The first thing a deck looking to maximize Ultimecia, Temporal ThreatUltimecia, Temporal Threat should include is evasive creatures
Rebbec could use some protection and it shouldn't be too hard to get an assortment of mana rocks and equipment out to cover most low-CMC slots
Additionally, the surrounding foliage and bright green leafage only make it stand out more, calling back to other classic Scarlet and Violet cards like Koraidon ex Special Illustration Rare from base SV
Yes, you can copy your own Doom before its sacrificed at the next end step
If the specific part of it scares you, then Im pleased to inform you that theres tons of these, for almost every strategy you can think of
Scuzzback Marauders Another persist creature with one keyword ability, at least Scuzzback Marauders has a better rate than the rest