Infantry tank

I would safely say that you can eliminate Adult Tiki, Brave Ike, and Chrom simply because you need some more color variation imo. You already have Loki who does substantial magical damage, and who also is a great buffer. So I’d say in the end you should either go with Beruka or M! Kana. I’d take Beruka personally just to check the Owain counters but M! Kana synergizes better with Loki’s buffs.

Lukas is great blue wall , he has easy access to merges and lack dragon weaknesess. Huge def stat + huge HP pool but non existent res and speed + decent atk.
But M! Kana is definately a fine mixed tank to build with default kit. He is more flexible in terms of builds he can run.
1) wall (default weapon water breath)
2)buffer (although it's a bit harder to use, weapon needed light breath + ref, buffs all stats)
3 debuffer (similar to buffer, needs to attack first to apply debuffs. Weapon needed dark breath+)

I still suggest Lukas in his case probably

You might not have noticed that Beruka is a flier. She'll work on the team though.

If you don't mind investing a lot into a unit that doesn't merge especially well, M.Kana is a pretty strong option. I actually consider him a better version of Nowi. Give him lightning breath+spd, QR, aether, atk smoke, and the rest depends how you want to do it. Steady/warding breath + close or distant def is going to do really well; brazen def/res is also fine; spd smoke can be quite effective because he's decently fast to begin with so with spd smoke he'll double a lot in PvE up to infernal difficulty even if his QR falls. Light breath + DC is also interesting with atk smoke, spd smoke, and flashing blade (pick your preferred two)