Yes, i had considered that the way it could work would be signing a great FA pitcher with Kikuchi then being the defacto 5...however that still leaves a problem with depth IF a trade of Manoah happens for a bat. Then you're still looking at scrap heap minor league deals where guys know they have virtually no shot at a major league job unless someone gets hurt. Not the worst situation to be in I guess, but when that's the case you don't get to pick and choose the signees, you get to pick the leftovers in January/Feb nobody else wanted.
I think in that kind of scenario it would make more sense to sign both a big name starter and another middling guy and trade Kikuchi for a bat while his value is higher, keeping Manoah to pitch at AAA alongside Tiedemann and whomever else they can scrounge up.
And yes Ohtani signing would be so huge. It's probably pipe dreaming, but the marketing he could be a part of for Rogers would be unparalleled