As one of the leaders in said community for the NA scene I wouldn’t lay the blame entirely on Splat3. Things were slowing down before then and a lot of the old guard were hanging up their hats.
Networking left much to be desired, but we also started the grassroots on the Wii U after all.
I adore the mork borg system, but similarly have been limited to one shots as well. But one thing that stood out was that just a bit more hp dramatically improved their staying power.
Characters closer to ~10hp could survive a whole session. So my thought was to run a DCC style funnel with MB characters and have the selected survivors get better right after. They should hit that ~10hp staying power while still being squishy.