For your requirements:
Ability to locate people for rock climbing but also for motor sports in the future, I would recommend the Canon EOS R6 series or the R8. These have the benefits of Face and Eye detection and tracking, and can lock onto faces in cars, which I suspect might be particularly helpful for motorsport aspirations. The R6 series (but not the R8) have In Body Image Stabilization, which is good for shooting hand-held and tracking. All three share the same full-frame sensor that is particularly good in low light.
You can use your EF lenses on R-series bodies via a Canon EF-RF adapter, and these work extremely well.
HOWEVER, you would have to consider abandoning the EF-S 24mm lens as it will not function well with a FF sensor - it cannot use the whole area of a FF sensor and thus reduce your pixel capacity significantly.
I would consider selling your T8i along with the EF-S lens to help fund the purchase of a R6 and one or more RF lenses.
While the EF 75-300 will work with the R-series bodies, it is considered one of Canon's poorest optics and does not itself have image stabilization. If you like you can continue to use it but I would recommend saving up for an RF lens that could cover the same range - my pick would be the RF 100-400 USM, or the RF 24-240 USM, which are both excellent optics. There is not such a hurry if you are not quite ready to do motorsport shooting.
Within your budget you could get one of these, especially good value as refurb units:
EOS R8 refurbished, body only
EOS R6 Refurbished, body only
As regards lenses:
Refurbished RF 100-400 IS USM
New RF 50mm f/1.8 USM
New RF 24-240 IS USM
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
