My gut feeling is that Mojave is going to be your sweet spot. Catalina should work on it, as @ashley notes, but you’d lose old 32-bit apps, and I suspect you have some. High Sierra will fall off the update train shortly, so that doesn’t make much sense.
There may be some difficulties in upgrading to an older version of macOS; it’s possible, but the longer you wait to upgrade, the harder it gets and the more gotchas you’re likely to encounter.