I think it's fair to say that the current implementation of the defender bandits is very barebones and one of the weakest parts of the mod as of now.
As it is now, defender bandits feel like a remnant from a indev build; they do nothing inside their own home, there's simply too many of them inside of the buildings they spawn in (a 25 person max I believe), and the defenders themselves look ridiculous.
Defender bandits, for some reason, do not scale in anyway, which is very jarring when 6 months into the outbreak you are exploring a gated community and find that one of the houses have a community of sports fans and generic citizens inside.
As I understand it, the defender's appearance and equipment-tier is based solely on the building they spawn in, so if they spawn in clinics, military bases, etc. they'll look different.
That's all well and good for maybe the first couple weeks, but there's a reason Desperate Citizens aren't seen beyond the first couple weeks.
My proposal is to have two separate variables that alter the equipment and strength of defender bandits.
The two variables will be building type (x) and time survived (y)
The way I'd imagine it working is that you give some value to the type of building they spawn in, and then give some value to time survived and multiply them together for a "strength level" which you can then use to determine how strong the bandits within will be.
This means, yes, on day 1 military bases will have soldiers on it since the building type's innate strength is high, but on day 160 neighborhood homes won't still have random civilians basing up, because the time survived will increase its strength. Maybe instead they'll also look rather militaristic.

You could hopefully apply a strength scale like this to also determine the amount of bandits inside a location, retaining that cap of 25, but making it a bit more difficult to reach it, as, again, finding a gated community that's completely deserted except for ONE house being used to shelter nearly 30 people is rather immersion-shattering.

Hopefully I made sense, and if there's anything that's unclear just ask.