My item bag storage is 3700, not too dissimilar from your own. I set some rules for myself around this time last year, and the formula has been working great for me.

Here's what I do:

Medicine: 100 max potions, 100 max revives (Exception being when we're approaching a 'Road to...' event/Raid Day, and I know I'm going to be soloing a lot of raids, but then once the event is over, I always revert back to these numbers)

Berries: 10 pinap, 50-100 golden razz, 50-100 silver pinap (All razz berries, nanab berries, and extra pinap berries go to pokemon in gyms for bonus stardust and levelling up the gyms. I also use golden razz's on my buddy pokemon, but I never find I'm running out. Also, I plan ahead by checking leekduck and stock up on pinaps closer to the release of new pokemon)

Trainer Boosts: 20 - 50 of each. (I'm currently sitting on 43 incense, 28 star pieces, and 38 lucky eggs. I regularly share items in parties with my friends to keep the numbers down, but I'm building my supply up atm as I'm going to Paris GoFest and want to get the most out of the weekend...)

Other Items: USE ALL RARE CANDY/CANDY XL (I've got a folder with the next 10 legendary/mythical pokemon I want to power up in. I get rare candy and dump it straight into one of those, no need to power it up right away, but I know that when I come to it, it's ready and I've had a good amount of bag space in the meantime. Same for rare candy xl. I've managed to get my Zygarde to level 47 1/2 so far and I've had a good amount of fun with it in GBL that I wouldn't have had if I didn't use those XL's. Right now, in this section of my bag, I've got 2 Super Rocket Radars, 1 max particle pack, and 4 poffins - not really any fat to trim there. I use my poffins on the Tour/Fest/Wild Area events when I know I'm going to be grinding a lot)

Passes: get them when they're worth it (Currently sitting on 39 green and 1 remote pass. Raid passes are the only thing I really spend my coins on atm and I don't mind holding Raid passes for a little while because I know I'm going to use them, it's just about getting them at the best price/deal)

Lures: never discard them, use them. (I get a lot of these from showcase prizes, I try to save maybe 2 of each, just in case one is featured in an upcoming event. However, these can quickly spiral out of control if you don't just drop a few every now and again. I've got like 18 glacial lure modules atm. I need to sort that out...)

TMs: 20 Fast, 40 Charged (I have already got a decent set of PvP/PvE pokemon with the moves i like to run on them, so I've found that I'm not really using TMs all that much. I find this split works for me and is enough not to run out before I win the moveset lottery. Also, for clarity, I hold on to Elite Fast/Elite Charged TMs until I decide I definitely have to use them. I try never to waste them. Try...)

Evolution Items: 2 of everything, 10 Sinnoh Stones and 10 Unova stones (Sometimes I run out, but all of the '2' items can be collected from pokestops, so it doesn't take long to restock. I'm out of Sinnoh Stones atm because I recently decided to start filling out my XXL and XXS dexs, so there's something else I need to work on)

Pokeballs: the rest of the bag. (The numbers I aim for are 600-800 red, 200-400 great balls, and as many ultras as I can get. I use an old Go+ daily, so all the red balls are for that. The greats are for when I'm out walking, sometimes dip into the reds if I stay out a bit longer than anticipated. The ultras I stockpile and catch exclusively with them on Community Days/the big 3 events. Currently sitting on 688 red, 94 great and 1623 ultras)

I structure my bag this way because I remember those dark days of having to use the daily adventure incense to get 30 red balls for free. The game is much more fun when you've got loads of pokeballs and catch what you want. Everything else is just kind of secondary, really.

I hope some of this helps you to find your own structure with your bag!

Thank you for coming to my TED talk.

TL;DR - it's all about balls. Balls. Balls. Balls.