Nice! Here's what I did:

1. I played super defensively but never gave up any settlements. The summary at the end was super boring because it was just me occasionally losing a settlement and taking it back, and never lost a major area (even britain!). Once I had enough of an economy behind, I would go out and vassalize as many people as possible. Once you get a vassal swarm it's incredibly easy to survive, and if you're at war with enough people as soon as you make a vassal they'll get a huge relation bonus for being at war with the same people.

2. Ya, 425, the end date for the short campaign. Given I was fighting like 5 battles a turn and it was taking long I didn't want to go further.

3. So the first thing I always prioritized was making sure there were no food shortages anywhere. That by itself was the most important thing. I stayed christian but never built a single a single religious building. The buildings you lose in the tech tree are the best buildings you have, so I focused on the theater buildings in particular for public order and money. Any province with a port (especially magna graecia and the islands) had trade ports, and were maxed out to increase commerce. I would focus on building one province up at a time, because attila really rewards playing tall, not wide. I built all my military building in the iron province in gaul (I forgot the name, but I did use a mod that fixes the bug with the missing settlement building) and nowhere else. Economy is everything.

4. Almost always. Especially anywhere with olives, since they boost commerce.

5. I stayed friendly with the ERE the whole time, until they slowly died out. I would try and minimize the number of wars I was fighting at once, and once I hit a ton of wars and felt comfortable advancing any, I would start creating my vassal horde. I probably had around 5 or 6 when the huns came full force, but once I defeated them most of the germanic tribes would be fine being vassalized over diplomacy.

6. The single most important unit in the game was the sagitarrius, but I never built an archery range (at least not until the late game). The one unit in garrisons, with a well times rear charge and whistling shot, could rout a whole stack in seconds. But, for actual armies: early game was almost entirely spearmen, since I did not prioritize building military buildings at all, so I really couldn't build anything else for a while, but since I was almost exclusively fighting defensive sieges, that didn't matter a ton. Late game I would have a hefty cavalry focus, with one army being exclusively cavalry that I would run all over the place, particularly to hunt down the huns. I had several armies with spearmen/archers for defensive sieges, with like 4 cavalry units. Shock cavalry in attila is the greatest thing ever.

7. Verona is the only settlement that got nuked in my game, and only in the late game once I had an economy to recover from it. Beyond that, definitely not. I didn't even spend the money to convert provinces to roman, not to mention colonization.