Which grail units make the best merge projects?

Hey there. Now that the new update will eventually make +10 TT and GHB units possible, and the grails dont seem to be doing anything else, I decided it would be best to pick one of these units to invest grails in. We already have 200, but judging by the video, we don't get a lot of grails per season, it seems to be about 20 per Saison (kind of like the amount of Refinement stones gained via Arena Assault). I do expect we get some extras per quests, but I assume the rate of getting grails will be about the same as earning Coins and Refinement stones. This makes picking the right unit for it, especially for your initial 200 Grails, really important! Here are a few thoughts on the matter.

Let's take Michalis as an example. If you played since launch like me and never wasted one of them, you have enough Michalis to +5 him without using any grails, meaning you still need 5 more copies of him via the grail summoning. The rate goes up by 50 everytime, so its 100+150+200+250+300, so +10ing him costs you 1000 Grails total already.
But let us note that being one of the older units, Michalis has been powercrept quite a bit already. If you compare him to Walhart, he has the same BST as him even though he is a cavalry unit. He also has more up-to-date skills. However, even at max you can only +2 him so far, meaning you need 8 more copies of him. Due to increasing grail cost, this now takes 2200 Grails.

If you think about cost efficiency, Masked marth is the best Choice as you can already have him +8 if you played since the first TT, so you need only 2 Copies at a cost of 250 Grails, which is almost covered by our initial 200. Overall he seems like the best start if you aren't put off by his lack of skills.

For arena purposes, I would say the best pick is Valentine Eliwood, since he is the only free unit with a BST of 175. Even the other armors like BK and Arden reach only 169, being two whole scoring tiers lower than him. However, Eliwood needs 9 copies, coming at a high cost of 2700 Grails...

And Eliwoods skillset isnt particularly high-scoring. I do wonder if units like Linus or Black Knight make for the superior merge project choice because they have a refineable weapon or unique special skill, even though they are in a lower scoring tier.

So, all these things considered - which of the units would you prefer to pick as a long-term merge project? Those that come at a low cost, taaking considerably less time? Or those with a high BST or high-scoring skills to help you in Arena? Or just characters you love? I would like to hear your thoughts.