The SP system in this game feels like it needs changes. You gain far too little for how expensive some of the skills are. In fact I'm pretty sure there's skills you can't even reasonably get enough sp for at ALL in one playthrough and they're not even gamebreaking ones.
You can earn SP books from the Tempest Trials and Relay Trials, I'm actually swimming in them now in the postgame. They can add 100, 500, or 1000 SP to a character.This is what I've been wondering. I've been saving SP all game (still haven't spent any of it on anyone) but it's because I thought I'd be able to get some really good stuff later on if I saved, but now it's looking like I'll never be able to afford something that costs 4000-5000 in literally my entire playthrough, which is absurd.
One way they could help get rid of the decision paralysis (other than just make skills much cheaper) is if they did it sort of like the Statue upgrades in Three Houses: Every skill you purchase from an emblem makes all other skills from that emblem lower in cost by that amount. Or... something like that? Maybe they could let you sell skills back? I don't know. Basically as it is the SP skill purchase system is far too restrictive and limiting. Case in point in particular being the stat-ups: I could buy Speed +2 early on and that'd be useful, but what if later I want to upgrade it to Speed +3 or Speed +4? I don't want to waste points on Speed +2 if it's going to be obsolete later on, so I'd better save my points, but then it'll take forever to save up for something like Speed +4 so the actual result is that I end up doing absolutely nothing and not interacting with the system at all. Which just seems like terrible design.
I'm not sure if you only unlock them after beating the game though, as I only messed with Trials after beating it. The Relay Trials I've finished do say all players get them, but I'm not sure if that's a display oversight or not.