Considering how Harvest moon works actually, you can't use it properly into a fight without drifting your rotation by 2.5sec at least, leading to further ressource problem. One good exemple of such problem is P7S fight. The boss send you twice into the air during the fight, cumulating several seconds of stun. This will ultimately lead to a skip on an Odd minute Enshroud in the later part of the fight. We can say that using Harvest moon or Harpe will create the "same" situation. (considering you have a theorical 100% melee uptime on the boss)
Not only that, but having only one use of that through the fight is kinda "meh". Of course you can use it as a tool to keep attacking from a distance without loosing too much potency because of the downtime, but that's only 2.5 sec of break. P6S is a good exemple if you aren't lucky with the Exchange of Agonies forcing you to go outside instead of inside.
Unless you don't have the choice or you don't optimize the fight (which will be very specific) you don't want to use it.
Now on what you propose :
The idea of a "Shoha-like" ability isn't really a bad idea. It can give some new life to the spell, and I think it will not be too much on the actual kit.
For the resource part, first of all, hard disagree with the arcane crest giving gauge. If we admit that the boss is actually doing a raid wide every 30 second, and that the reaper is using arcane crest 100% right this will give the reaper a mere 2.66 Potency per second boost (which means 6.66 for every GCD of 2.5). And that's for the very best in theory ! Which means that the true value of this will be consistenly lower. (Based on a 10 minute fight)
Imo, Samourai face the same problem and I think it's not normal for a class to have a ressource generated depending on what the boss do, affecting your performance overall if the boss isn't doing any raid damage. Samourai need to use third eye 3 time then 2 time (repeat with 3) correctly to unleash more shinten, which takes a minimum of 75 second for a 540 potency (through 2 attack). Getting something near 18 Potency per GCD (in theory), which is still 3 time greater than what the reaper could have.
Your idea on shadow of death could be of some use, if the general idea behing the RPR wasn't to keep it up all the time. And sadly, it's not the case here.