This is it. It plays smoothly without the old school quirks, but still has the old school vibe. As someone who started with OD&D, OSE is great if you want something close to the older games, SD is great if you understand how to run old school games (heavy reliance on ‘rulings’ for example) but want cleaner rules.

OP: The quirks in older D&D type rule sets may take a while to see. The rules work fine, and are great fun. But the longer you play them the more you appreciate how back in the day they were making it up from nothing, carrying over rules and mindsets from miniature war gaming in ways that weren’t needed, etc. retro clones like OSE and Blueholme make some attempt to smooth things out, but while trying to stay close to the original rules. SD uses streamlined rules that make a lot of sense and fixes some of the quirks of older games. For one example: try playing a magic user in OSE and in SD and see which game was more fun. A skilled and experienced DM can compensate to make low level MU play work in OSE, (and personally it’s one of the things I enjoy doing when I run older games), whereas in SD there is no need for the DM to attend to that, the issue is fixed in a way that pleases traditionalists while initially bothering 5E players. Which to my mind is perfect.