there is a stark lack of units who I would define as being tanky in Birthright. the closest you'll really come is Rinkah (who's growths are closer to say... Eliwood from Blazing Sword than anything that can really stand up to hits) and maybe Silas for the early game. I'd highly suggest making Corrin's sub-class to be Knight because of this, and just burn any boots you've gotten from anna gifts/path rewards on Corrin to keep them somewhat mobile so they cna take hits.
Don't worry about Corrin missing out on skills from Hoshidan Noble either. it's SUPER easy to get skills for Corrin and others by using the top ranked My Castle searches to find skill grind maps. . you can buy a skill off of a unit for half price if you challenge one of these straight beelines to the throne with no handicap, and then that unit will get put into the logbook to hire as an einherjar or learn the rest of their skills from.
For that matter, Mr. Laser, if you plan to do anything more than just run through the campaign of just Birthright, I HIGHLY suggest you pay for the money map, Ghostly Gold. mastering this somewhat randomized map with safe strategies will save you SO much time on maximizing weapons, hiring Einherjar, and making sure you can afford to buy expensive skills, etc.
finally, I'd like to share the Einherjar Maximization trick. I know i've talked about them a lot and it seems silly to invest so heavily into units that cannot support, but hear me out...
First off, when you beat the game, you can logbook 5 of your units for other playthroughs. they'll have the stats and skills you've given them, just not the items they had on them.
while it's entirely possible to max out a regular unit through gameplay, you are at the mercy of the RNG for this and have to keep buying eternal seals and gaining EXP at a snail's pace. Royal Royale DLC can provide stat-up items, but... good luck trying to win that map consistently! even the stronger teams on that map struggle to achieve consistent victory, I find!
Einherjar are INCREDIBLY easy to cap out, though. They become Excellent fall-backs for when it's time to money grind or when you just want to burn through the story or such. they're expendable as far as the game stats are concerned.
To max out an einherjar, you will need 2 save slots, preferably one of each stat- up item (or the combination of Dragon herbs, Boots, and a Seraph robe) and you will need around... oh, 80,000 gold just to be on the safe end.
Step 1.) load your file and go into the unit logbook in my castle. this is curated by the NPC who stands right there by the throne of your castle (Jacob/Felecia by default) and hire the desired Einherjar you want to become a BEAST. note if you've already hired the Einherjar before, you need to update them in the logbook and dismiss them and save before continuing.
Step 2.) feed all the stat ups/first blood/class change/skills/whatevs you want to give to your einherjar.
Step 3.) update the log book and save your game into the other save slot.
Step 4.) reload your original save. rehire your einherjar, feed them the same stat-up items you fed them and update them in the log book again. overwrite your secondary file again.
What this is doing is that the Log Book is part of data that is shared across all saves in Fire Emblem Fates. if a save occurs to ANY file, it updates the logbook universally, but will not say... update other copies of the original file that you burnt the stat up items on.
I used this for Birthright towards the late game because while i payed my dues proper in conquest and played that game (for the most part) like a proper fire emblem game, I just got sick and tired of Birthright by the end and wanted the free Einherjar of my units.