CastOff - Fix all systems to 2, 4, or 8 measures (casting off)
After you've finished note entry and are ready to think about layout, enable this plugin. It'll automagically (do its best to) fix all systems to 2, 4, or 8 measures (you can choose!). What's more, it stays on after that, so you can start to add your own system or page breaks, and it'll continue to adjust the rest of the score as you go! Basically, from that point on, you can treat it like a drop-in replacement for the built-in layout system.
Right now it's only for MuseScore 3. But if there's enough interest, once the 4.x Plugin API stabilizes I may look into an update.