Basically, the calculation is really secondary compared to games and is mostly Python and C++ related programs so I might have overestimated how heavy it is, 4 cores should be fine as long as i don't do too many other things while those are running.
As far as TF2 goes, goal would be at least 200 fps in 6s with a 1920x1080 resolution on fairly high graphics and around 120 in pubs with the same settings, while for those other games, since I would like to play on reasonably high settings in 1080p while maintaining around 100 fps, I'm not really sure what I'd need. I would assume 1060 is about enough looking at benchmarks but maybe a non-Ti 1070 would be the right compromise between a too demanding 1070 Ti (in terms of price and having to get a better mobo and CPU) and a now fairly aging 1060 ?
Here are the two builds :
https://www.materiel.net/configurateur-pc-sur-mesure/af1fd572e2e9e0ddee6db2dd7e7d0c75/ - 1060 6Go
https://www.materiel.net/configurateur-pc-sur-mesure/68de30d4b941e215c82918e6340d4aa8/ - 1070 8Go (non-Ti)
Hope the 1070 one is coherent (meaning the CPU and mobo are fine for a non-Ti 1070), and I wonder which one would have the better performance to price ratio with the game settings I described earlier in the post...
Thanks again for spending some of your time to help the community !