www.makemkv.com

I've been ripping Blu-ray ISO files in Makemkv. A vast majority of the files that come from the ISO files are in h.264 and, from what I understand, lossless in quality from the blu ray iso, and so a lot of them have large file sizes. If I were to convert them to h.265, would I lose much quality? I know that going from a lossy compression to a lossy compression is not ideal, but considering that my starting files are ripped straight from the blu ray image, would I be making a lossy copy of a lossy copy, or would I just be making a lossy copy of a pristine original which makemkv produces?

That's a question for the authors of your program to do the conversion - MakeMKV is going to output the same codec as the original.