You are treating Reaper Form as a primary movement skill, which is not its primary role. The movement aspect of it is just a secondary aspect of it.
Reaper Form provides you with basically twice as much eHP and other offensive and defensive properties.
I havent played in a few months but I remade my Decay Lich from 1>93 last I played and think its one of the strongest builds in the game still and thematically the best made class due to the HP drain and constant need to leech hp
I dont use transplant, dont need it. I use Deathseal which procs Hungering Souls, dmg reduction and Channel Wandering Spirits manually against tanky rares/bosses
Wandering Spirits ‘nerf’ wasnt a nerf at all, it was a gigantic buff at the cost of autocasting which is actually replaced by Hungering auto procs anyway
The build becomes so tanky with Deathseal/Titan Heart/Endurance 60%, as long as you have the Threshold to match your 33% you are permanently taking massively less damage and even having 700hp available, you basically rarely get taken out of Reaper because of the endless recovery from perpetual dots applied to everything
edit: on my setup if I am leeching constantly I can remain in Reaper Form for minutes, theres no monolith thats too long and you wont fight a boss so long your form drops unless you didnt resummon into right before the boss, thats why I dont use transplant because if I leave Reaper form im 95% dead anyway
Then i can reformulate the goal of my proposition as : allow Reaper form to become (with its skill tree) a primary movement skill (so Transplant is less mandatory) by giving a way to reduce its dowtime while balancing its defensive benefits.
A possible way to do that in an intersting and thematically fitting way (I think) is to reduce the dowtime to something like 10sec and make that spending life for skills can reduce it further, but make that the healling to full life when going back to normal form takes some time to kick in (the life you get on normal form depends on the time you spend in reaper form). This way, the dowtime to the defensive benefit is still there (and could be interacting with regen or healing stat to make it more interesting).
I don’t think you’d be giving up enough for what you’re proposing. You’re cutting the cd in half in exchange for delaying the healing when you switch back. If you cut the healing, you’re paying a price for the 10 second cd reduction, but a delay is negligible considering how many leech mechanics the lich already has. Trading an instant heal for yet another heal over time is nothing.