Master seals simplifies promotion, and opens up extra rom space for additional items, so unless you intend on patching in an extension to open more space, I’d recommend them over class specific, plus it allows more flexibility with party composition.
I like the idea of balancing around bases, growths, and availability, but I see potential issues; for example I’m going to hazard a guess and say Joseph is a Sety archetype, and ultimately represents the interests of his nation while his brother tries to maintain the peace in their newly stabilized nation, as a result, he’ll be a psuedo-lord of sorts who rejoins the plot fairly early after the time skip. Compared to brett, who comes in with a sturdy base HP and Pow as well as the stronger fire magic, Joseph relies on growths and the high hit/crit of wind magic… but he gets a majority of a chapter alongside Miles to play catch up, can potentially get the energy ring to patch a potentially lower pow, and gets forsetti and S rank wind magic like Rexcalibur or Blizzard or Fimbulvetr to become a critting machine. So unless Brett gets a significant chunk of additional chapters to shine, he’ll be overshadowed. Admittedly, Brett has fire magic with its own checks to take advantage of… but then so does Lael (hence my suggestion)
I do like how you’ve balanced everything; it definitely skews towards challenging, especially with the reinforcement spawn mechanics, but I like how it gets me to react and adapt and better utilize under used weapons. I’m curious to see how you continue the balance from here, and how you’ll contrast Faith from say, a wyvern rider you recruit, or her rival/sister you run into in Ch 9
Caveat-ting of what Voni said, I think the issue Leah runs into is she is an Elrika by design, especially with her growths and bases, meaning its a bit of a coin-flip how good she is. I like the relatively early angelic robe to give her the extra HP she needs (although I would change that back to +7, but I digress). What would help her, potentially, is a buffed con to mitigate sword weight so she could compensate for a potentially lower Pow with the various iron blades and steel swords that drop. I could also see a prof weapon down the line to invert weapon triangle and with a 15 crit to help compensate as well. Maybe a gift between nations to strengthen peace negotiations?