You’re not exactly a “common” user here, my guy… you care about security.
I’m no help on the LUKS front for my aforementioned reasons beyond pointing you to the OpenWrt forum but your experience there is going to be a heavy case of YMMV. The latest stable firmware for the Slate AX is still built on OpenWrt 21.02; the latest mainline OWRT is 23.05.x.
If you really want mainline OWRT then you’re going to end up looking @ @solidus1983 's vanilla builds for the Slate AX. That’ll mean you’ll be dumping the GL GUI (oh well) but more importantly the closed source SDK they use. There’ll probably be a performance hit in doing so (not that I’ve yet tested it).
Blah, blah, blah… the point is: if you want full access to the OWRT ecosystem you might want to consider dumping GL firmware & just eat any performance hits. You still won’t have anything beyond LUKS1 though.