Personally I think the only really tenable way ahead for a long term solution is for Continental and Lycoming to create as close to possible a drop in diesel replacement engine for the turbo 520 and 540 series engines. Yes there will be an initial wrenching switch over but 100 octane days are done. When not if, 100LL is euthanized by government fiat or a supply chain failure, I don’t believe there will be any viable affordable replacement and the small and decreasing volume of 100 LL pumped just is not there to interest the deep pockets who could put the kind of resources to solve the problem of how to get 100 Octane without the lead.
94UL is a proven existing solution for the smaller engines but Jet A is the future for GA high performance piston engines