Trails works already - adjust the release time and it will slowly fade out over that duration.
Are you sure mix isn’t assignable to expression already? That seems unlikely, but I’m not sure.
There’s “dry gain” and “freeze gain” which can both be assigned to XP, should be able to control mix levels from there. I usually put a rev or delay after the Freeze if I need LONG trails/slower fade-outs
that’s what’s great about these forums and groups, the QC is so deep- none of us can know it all! These are great resources for anyone looking for workarounds or solutions.
A moderator will probably decide what to do with this thread. It’ll be a good archive for anyone with the same question in the future.