beagle1

crop factor won't matter

no, what are you trying to buy ?

I don't think your primes will become "obsolete" anytime soon but zooms are more versatile.  You still use older EF lenses with Canon mirrorless, the adapter is just a metal ring with contacts

For birds you're going to need something longer than 300mm -  e.g. 100-400 or 150-600 or a long telephoto prime

I use the older 70-200 2.8  "ii"  on mirrorless If they have a return policy and one year warranty go for it

shooting RAW or Jpeg ? if jpeg what is the "picture style" profile settings

I'll vote Canon 15-85

my thoughts:  go mirrorless -  Canon R5, R6, etc.  which can use older EF lenses

all would probably be better  (AF, MP's, etc) than a 10 yea old camera.   And mirrorless is smaller and lighter and can use the older DSLR lens. you can do a side-by-side comparison   - https://www ...

For birds and wildlife you would need to upgrade the lens -  e.g.  100-400 but much larger and heavier than the 55-250.

not really, the next step up in IQ and reach is a prime

OK - whatever ... didn't click the vid link I don't experience the "pulsing' or "hunt" with my 150-600  but I think the 100-400 probably has better/faster AF www.flickr.com/photos/mmirrorless

The Canon R adapter is just a metal ring with contacts - works great with older EF lens www.flickr.com/photos/mmirrorless

sorry, I'm not aware of any "pulsing" with my 150-600  -  whatever that is www.flickr.com/photos/mmirrorless

full frame is definitely better for indoor sports -  also consider the cost of lens

??

I use both the Sigma 150-600 "C" and 100-400 v1  on a Canon mirrorless I prefer the 100-400 for the smaller size and weight www.flickr.com/photos/mmirrorless

Canon R6 and 100-400 and 70-200 2.8  are good for wildlife & people

get closer with the 70-200 2.8  :-D

I'll vote 100-400