For codecs it is highly dependent on the release group. For 4K it is the only valid option, but for 1080p a lot of groups make their x265 encodes too small and sacrifice quality. Take a look at the group rankings in the trash guides for sonarr and radarr for a general idea if who is the best/worst.

As for Tdarr, you should only really use it for audio and subtitle processing. For one you should not re-encode video so unless you're starting with remuxes you're further degrading video that is already degraded. And for two it's best left to the people who know what settings to tweak for each movie or episode. There is no universal setting that works well for everything so while you might be able to get acceptable quality with automation it's never going to be great. The best groups already took the time and effort to get it right so you might as well get those and save yourself the time/electricity.