beagle1

indoor sports -  Canon R6 and f2.8 lens wildlife -  at least 400mm -  e.g. 100-400

you have a a sony a7iii and you want a "sub" $100 lens .... :-D

yes -  but I don't have either lens

I used to own the 400mm 5.6 prime years ago - sharp but I ultimately preferred the more flexible and versatile 100-400 the 400 prime should work even better on mirrorless cameras with IBIS  ---  ...

I vote Canon R6 -    I like Canon colors and "IQ" and the R6 can easily use EF lenses

I would like a 600mm - beyond the budget :-(

LOL All the camera manufacturers need to "step up their game" because I need a 500g  50-600mm :-D

I think that "angle finder" works on any camera with a VF but you can straighten ... "level" horizon lines in post processing  :-)

I think all the newer mirrorless models have electronic shutters - e.g. Canon R5, R6 Sony A9

IBIS and "eye" works great on non-stabilized lens

For birds  Sigma 150-600 "C" but be forewarned -  it's big and heavy Canon 100-400 smaller and lighter

smallest,  lightest and most compact interchangeable lens camera -  Canon M6 ii with M and/or EFS lens

The Canon R5 can easily use EF lenses with adapter For landscapes the 15-35 or EF 16-35 or a wide prime

I'll vote full frame mirrorless

everything will be a significant upgrade Mirrorless like the Canon R, R6, etc. can easily use DSLR lenses with adapter

I currently use the Sigma 150-600 "C" for birds and wildlifee but the smaller, lighter 100-400 gets more use www.flickr.com/photos/mmirrorless

you can compare lens at www.opticallimits.com

I have the Yongnuo 35mm 2.0 good value !

you can try extension tubes better-   100mm 2.8 macro (non-L) for around $250 used

the only 'cons" to mirrorless is the cost - otherwise mirrorless cameras are more accurate and faster auto focus  "AF", and higher FPS and smaller and lighter and mirrorless can use all DSLR lens ....