That would be one of the easier balance corrections, but it wouldn't fix the issue.  What if a player attacks you with 200 Marian Roman Legionaries? That are magically stacked in a corner of your Fortress. The end result would be the same. No, the enemy needs to surround the Fortress, and you can't surround it with 200 units.

Remove disbanding formations… thereby turning formations into battalions. Bad idea. I personally have never encountered this exploit everyone is mentioning, and it doesn’t seem like it would be much of a problem if we just fixed collision.

We have done that on Classical Warfare AEA.  Lowered dmg a little, but garrison is 35, up to 50 if you get the tech.

Another potential solution that came to mind is increasing the number of units that can be garrisoned inside a fortress. Isn’t 20 soldiers too few for a fortress? Historically, 20 soldiers is already a very small number, it feels the same in game as well. A temple can hold 20 soldiers too. A fortress and a temple having the same capacity doesn’t feel right, maybe this could be adjusted. (It’s also interesting that theaters, which historically hosted performances for thousands of people in ancient times, can only hold 5 units.)

I expected (before knowing about capture mathematics) that capture points of garrisoned units value with a bonus factor.  Defenders with fortification fight from advantageous ground so one soldier is of higher value. This would be one option to strongen Buildings. Another possiblility would be to give a malus depending on the amount of attackers. So that capture points can not be massed the way it is possible now. Like the slower building process with more simultaneous workers. (Just my 2 cent....; but yes at the moment 0ad is an attackers game)