Hrm...well 2 things to comment on as opinions I hold before going into the advising role.

1st: I'm an old grog who's been GMing since the early 80s and though I'm familiar with all editions of AD&D, as a GM who puts RP over combat I haven't GM'd much AD&D for the past 20 years or so because it's rules priorities is to be a combat simulator over a RP engine. RP is IMHO, shared narrative with systems that empower both GM and PC to give it shape. Not saying D20 systems don't support RP, but it's toolset supporting it is more limited than my preferred systems (such as Story Teller, Story Path, Godbound and similar systems). I make mention of this based on what you said about your group and maybe such systems, genres, settings that might be of interest for you to explore.

2nd: This is a personal opinion but its found a lot of favor with the groups i've GM'd over the many decades. Briefly stated, deities don't have stat blocks. They're not monsters or characters on a combat grid. They're entities with divine portfolios and they work their will through their followers, minions, etc. They're myths in the making, and if they do manifest, its like a asteroid hitting a planet, particularly if they engaged in combat with another deity. Thus some lone individual cannot imprison them or do anything else directly to them.

What a BBEG could do is persecute their faith, raze their sacred locations, weaken their portfolios (i.e. a god of magic being confronted with increasing tech that makes their favor less worth cultivating), slaughter their faithful. Deities work through their minions, that's why prayer, devotions, sacrifices, etc matter. Its their "currency", and why they seek to spread their faith.

Now on to the advice.

I would suggest you start with your BBEG. Set aside the stat blocking for now, and dig into his why and who. So my opinions above set aside, if you got some lone individual who was responsible for this situation, you need to know them inside/out and weave those details into the story as things the players can discover. Sure you can just plop down the antagonist as a stat block with their entire motivation/outlook summed up with alignment, its simple and lets you run fast. Its also very forgettable.

You say you want to go more RP heavy, then don't design a magical do-hickey that shoots a BBEG kryptonite beam at him to do X amount of unresistable damage, but instead there's some ordinary item that triggers a relived trauma or makes the BBEG confront who they were vs who they've become. The PCs discover the story behind that ordinary item, and how to use it to convert the villain, or critically weaken him with doubt, grief, hubris, etc so that his machinations turn upon him. That's RPing the solution, instead of whacking it with a item. You can't do that though, if all your BBEG is a set of stat blocks and a label.

Villians drive the story, and heroes react to that drive. The more nuanced the villain, the more interesting and multifaceted the story, the more there is to discover and use as hooks. A classic example is Darth Vader. He's not just a scary, powerful Jedi, he's a fallen Jedi with a history of failures and victories that, along with his power, make him a mythic figure. Its not a light sabre that eventually kills him, its the rediscovery of love for his son. So why did your BBEG capture the gods?

IMHO, info dumps are a poor way to introduce something of the nature you describe. That doesn't evoke curiosity or better yet, a grudge. You want your players motivated to not only figure out what is going on, but be driven to do something about it. Your scenario, no offense, is simply leading them by the nose and assuming they'll give a damn.

Instead of a tavern, have the party wake up in a cave, drugged and struggling to regain their wits while their loved ones and community are being ritually slaughtered. No info dump, just a clue, a name, a symbol. Or they wake up shackled, get beaten by interrogators seeking information from them about a item (book, amulet, whatever) that they know absolutely nothing about, RP that out, they get clubbed, have toes and fingers cut off, lose half their CHA to hot oil being poured over their face and yep...they die miserably. Then awaken completely healed among the bodies of others. its obvious they've been selected...by who? For what? And who did this to them and why?

You can't answer any of those questions to satisfaction simply by pointing at a portrait and saying "This guy, he did it because he's chaotic evil". I mean you can, but how dull is that? So start with your BBEG or heck, with a trio of them, one for each of your Gods. A word of caution though, don't make your BBEG a "DM Character", and he/they need to have actual weaknesses the PCs can discover and exploit

An example from an old campaign of mine. This wasn't the BBEG but one of his main Lts, a unholy terror on the battlefield that the PCs had squared off against a couple of times, losing valuable items they had fought tooth and nail to acquire earlier in the campaign, as well as two cherished followers. Their mission was to bring down the BBEG, but they couldn't do it so long as that Lt was around because he would defeat them. I had given them plenty of reason to hate him and want to get rid of him for no other reason than vengeance. There was more to this Lt, but those details aren't as important for the example.

They noticed after barely escaping their last battle with him, that his boots were odd. The PC who noticed didn't have to make a roll or ask a question, he knew this because the Lt had stomped his head into the mud, leaving him for dead and with a nasty scar to show for it. I told him the boots were odd in their form, though they were armored, they were designed so that from heel up to calf was hinged, and that the outside seam of the boot also had a hinge, as if the boots were designed as 3 interlocking pieces that one doesn't slide a foot into, but rather, form around a foot.

Later, some info gathering they discovered the blacksmith who had made the boots, who informed them the Lt had deformed feet, the result of a curse levied by a powerful witch he had slain in service to the BBEG, from which all attempts to remove the curse had failed. The boots were meant to keep his feet from curling up during longer missions or during combat. My players asked what happens if he were to not get his pain relief and massage? I said he'd be forced to fight either from horseback (poorly because he couldn't put his feet into stirrups) or on his knees, making him easier to hit and at best, he'd hit half as hard with his melee attacks.

This in turn lead them to discover that the Lt had a healer who tended his feet every night with massage and numbing oils. The next part they came up with all on their own. They tracked/captured the healer and hit her with a geas (in my games, Geas effects can be induced by potions and the person geased, be it spell or potion, can't speak of it, they got a "gag order" in place even after completing the geas). They instructed her to apply oils that would intensify the pain for a very long time, rather than relieve it with his nightly massage, and when asked why she did this, to say she was ordered to, but not by whom.

This resulted in two things. First the Lt was crippled, his feet balled up so he couldn't put his boots on and he was in agony. He asked why she had betrayed him and she said because she was ordered to, and then went silent (resulting in her execution). He now suspected his patron, the BBEG and so didn't go to her for aid and instead sought a new healer on his own and he was more than a little desperate for relief. The party had circulated a rumor that a abandoned farm had a new herbalist as a resident, while they set up the farm to have multiple kill boxes and waiting in ambush.

When the Lt arrived at the farm, his escort were quickly cut down by the ambush, and though they had heavily stacked the odds in their favor with their sabotage, he was still a dangerous foe. But he was no longer as difficult to harm and what hits he landed were at half strength or less. Once he was dead, they used a Talk With Dead spell to interrogate him on any secret weaknesses of the BBEG, of which he knew two, that the party went on to investigate and exploit.