Tomes of Soldiery

So besides gear? What else can I do with these soldiery tomes?

You can trade them in for crafting mats and items you need for the relic quest.

There are mats you can buy with Sol, and the relic weapon has multiple stages where you either need Sol, or in the case of Alexandrites can optionally help you.

For the third relic weapon upgrade, you need 450 soldiery tomes for books to upgrade the weapon. I think it's a total of 9 books for a total 4,050 soldiery tomes.

It's 500 per book, so 4500 total for books.

You also need soldiery (and lots of it) for buying Alexandrite maps in the next relic stage if you want to earn more than one a day without having to grind FATEs and/or hunts. And in one of the stages after that you need an extraordinary amount of soldiery. But if you're not at those points of the relic quest yet, there's not a great deal you can do to dump excess soldiery... I guess you could always buy a bunch of Rowena's Tokens in anticipation of getting other classes to 50 so you can buy them weapons, or buy gear for your retainers, but unless there's any gear you still need, buying crafting materials is about the only thing you can really use them up on.