I've always shot with Canon body's, but have owned lots of 3rd party glass, which I really liked. Plenty of Canon glass too. Agree with everything that was said above. I prefer Sigma over Tamron, more for design and function over IQ which I fully acknowledge the G2's have. After moving to MILC, I thought I would transition slowly to RF. For still photography I think adapted EF glass works well, but I determined quickly that my Sigma lenses worked well, but not as good as native RF glass did. I also found out that AF on Tamron's 24~70 G2 didn't work at all in Cinema mode on the R5 C. Today I have fully embraced and am completely satisfied with my accelerated switch to RF.
Adapted glass can work, but in the end, RF glass with R body's is the way to go. I have the 150~600 C, but the 100~500 even at f7.1 is so hard to beat. Add a 1.4x TC (even if you can only use 300mm and above) is still worth it over the weight alone of the Sigma. On my wish list is a RF 100~500 mkII or introduction of the recently patented 200~500 with built in TC. We will see, but I would not hesitate going Canon RF over adapted Sigma. I think you will be more satisfied in the long term with this decision if your budget allows.