First things first because it's important and because I have nowhere else to put it: someone has to take one for the team and pityfuck Odin, preferably sooner than later, and it's non-negotiable. Ophelia's paralogue gives the best tomes in CQ and it's the only way to get them now with no 3DS online. You're playing a different game of CQ if your tome users don't have Calamity Gate or Horse Spirit, no matter who they are.
>Early Heart Seal?
I usually wait until some point after Port Dia if +Mag Corrin. Nohr Noble's growths are solid, the other options for early skills often aren't gamechangers, dragonstone has a solid matchup into Hinata, Oboro, and...everything, if you need a nos tank Odin is right there and needs a crumb of pussy to make Ophelia real, and tomes are one of the easier weapon ranks to build even in the 2nd half of the game. If you're trying to be fancy with HP management for Silas's Vow of Friendship so he can stomp earlygame, dragonstone is one of the more consistent, flexible, and forgiving ways of getting and staying at =<50% HP.
>What Corrin Talent?
Wyvern
You simply can't go wrong with Wyvern. It's first in the list because it's just #1. Three Houses gets made fun of a lot for wyvern dominance, but 3H wasn't the first. It has all the stats, all the skills are amazing, complete weapon triangle control with tome/axe/dual club, it's great in every map of the game (especially the worst ones), every unit wants wyvern if they can get it. It always just werks and feels good to use. I know I just did a schpiel about not early reclassing Corrin, but Corrin doesn't hate being in wyvern ASAP to start building axe rank for bolt axe and dual club. If your waifu is neither Camilla nor Beruka, MCorrin in particular benefits from it because his only friendship path to it is Percy who takes a while, compared to FCorrin who has a fast friendship with Camilla. Since wyvern is so easily available for FCorrin, she's more easily able to combine it with skills from less available, more specialized talents like...
Diviner & Monk/Shrine Maiden
They both promote into Onmyoji for the obvious Tomefaire +5 moneyshot, but there's more to it than that; Rally Magic is both rare and valuable in CQ, and Onmyoji gives the same kind of minmaxed backpack bonuses to spd and mag that a berserker does for spd and str. In a nutshell, it has all the different ways of enabling shitting out Mag dmg in one convenient package, and it gets more valuable the more tomes users are being used to benefit from it's rallies and pairup, so it's an engine for itself in a way. The differences between Diviner and Monk is that Monk's pairup bonuses are less minmaxed than Diviner and it can only use staves before promo, blegh, but it doubles down on rare and valuable CQ rallies with Rally Luck (boosts displayed hit, evade, and crit avoid by 4); Diviner can fight unpromo'd and gets +2 Mag though, which just werks. I think they're equally good. One gets Priestess and the other gets Basara; I don't have firsthand experience with them so won't say too much, but I've heard Priestess has good bases and runs a Shining Bow well (gets Archer from Mozu, Kinshi Knight from Azura, Bow Knight from Selena, Adventurer from Shura/Niles/Nina) and that Basara is a respectable mixed bulk tome user that enables a lot of saucy shit with Quixotic, such as...
Samurai
Tomes love Vantage, Life & Death, and Astra if you're saucy. Vantage builds tend to go somewhere in between one of two extremes: nostanking or vantage sweeping. Kilma in the Ice Village is a good primer of how annoying it is for most units to deal with Vantage + Nos, which Corrin uses particularly well from their natural bulk, insane class access, and unique backpack bonuses from Felicia/Jakob/Gunter. Weirdly, it has issues if you don't hit a sweet spot with damage; Nos wants to either one or two-round, and if your first round brings them to 1 HP, that screws with your survivability in round 2, so damage stacking from skills like Life & Death is something you're better off going all-or-nothing for oneshots with. That's an effective strategy, effective enough to steamroll some of the scariest shit in lategame even, but getting there and executing it can be minmaxxy, scary, and involve a lot of number crunching and trial and error for non-giga sweaties.
If you're saucy, instead of damage skills, you can gamble for oneshots and run Quixotic with things like Rend Heaven, Vengeance, Astra, or Dragon Fang from Nohr Noble. Which I should also mention. nvm im retarded and forgot you're not getting both vantage and quixotic at the same time
Nohr Noble
Gets maligned a lot by Optimizing Oswald for being a generalist class in a game that favors specialists, but it's competent enough to do real enemy phasing on it's own and really surprised me in my no reclassing run. Depending on how you play, sometimes it's useful even in the lategame to simply equip dragonstone and tank. Yeah, the debuff from Dragonstone+ sucks, but the dual strikes off it are fat as fuck while not proccing debuffs. Nohrian Trust is great fun even without aforementioned Quixotic, letting tome Corrin use Sol or Astra (they don't stack, only one or the other will proc at a time) from a backpack without burning a talent on character creation, a seal, or levels. Sol is pretty ubiquitous, even Felicia has Hero, but Astra is only coming from Odin, Kaze, or whoever they birth.

It's gimmicky and degenerate gambling, but it works and is unique and cool.
Dark Mage
Dark Knight and Sorc are excellent, but hard for me to justify as a talent when Odin, Nyx, and Leo are all right there. Though if I remember right, one of Fates's weird reclassing mechanics is that any Dark Mage that a DM Corrin marries gets access to Diviner (not Corrin though), therefore Onmyoji and Basara, which is worth mentioning. It doesn't show up on this thing though:
https://soapy4159.github.io/ferevpairings/ which is what I mainly use for buildcrafting autism.