Without Epic getting involved, there literally is no fix. Are you asking the dev to resolve how lumen GI accumulation works and build their own denoiser? Neobards entertainment would be capable of doing something like that?
The problem is Epic has their own upscaling and they won't bother fixing someone else's problem. They'll just respond with "Use TSR please? K Thx Bye".
Hopefully Sony's 2026 implementation solves this entirely, but until then, there simply is no PSSR fix to work with Lumen GI imo.