Sorry for my long absence, it was like a time warp... I was trying to handle several things but it appears I'm obviously a single-tasker so I lost almost a year. But finally music became the ultimate priority so just saying your advice isn't lost, and each bit of it matters, so thank you for that.
I had no idea rendering doesn't need the interface. I remember I bought Scarlett 18i20 precisely because I wasn't satisfied with render quality - losing depth in output file compared to the playback in Reaper. Maybe my thinking was wrong, but I was getting lots of cracks during the playback, so a dedicated card was a must anyway. Maybe my thinking was wrong again, and it was rather a CPU/RAM issue. But we had a small family band (RIP that unfortunately) so multiple inputs were tempting. That's just a brief history