[Harjit Singh]

The third party lenses (Tamron or Sigma), though very good & economical, said to have some issues with the auto focusing system of Canon R Series Cameras. So ensure good compatibility before purchasing.

[Harjit Singh]

Then, you may do a good research online for extensive reviews on these lenses & their performance with Canon R8 by pro wildlife photographers. You can also consider Canon RF lenses like RF 100-500mm f4.5-7.1L IS USM (though quite expensive) or RF 100-400mm f5.6-8 IS USM (quite economical but slow & usable in good light only) and may use them with canon Extenders 1.4 or 2 to achieve the desired telephoto range.