(This is not a paid-for/affiliate link post, I just had great difficulty tracking down a working solution for this so I thought I’d post the answer now I’ve found it!)
I tried a lot of things before finding the WorkOutDoors App. It lets you save a segment of a map (of any size) to your phone in complete zoomable detail and will even show a track of where you have been and a persistent compass pointing back to your start point. $2.99 on the App Store. Went from wanting to run in a straight line and back in unfamiliar places to feeling confident to go wherever after one run with it.
Some of the things I tried that didn’t work even remotely well enough: maps.me (app never appeared on phone), Sygic (one of many apps that wants you to plan a route on your phone and then download that and only that to the watch – didn’t even manage to achieve that), Yandex (seemed like it might work but only has Russian cities), ViewRanger, Apple Maps after scanning around the map a bit while online so it’s in memory (this was not totally useless but you can’t zoom).