I’ve got a LUMIX G9, which is probably the cheapest’pro’ level M4/3 mirrorless camera out there.
I tend to wildlife photography, so the G9 is not the greatest at Birds in flight but that is probably one of the hardest subjects to master. I haven’t tried any mtb action photos yet (but must do) it has a 6K photo mode which is very good for action shots.
Being Micro 4/3 all lenses are double the focal length due to the sensor crop, so my 100-400 is 200-800 35mm equivalent and in a package that only weighs 1.6kg, it is great for hand held shots and soo much lighter than a full frame mirrorless equivalent.