AR-750 Slate is a brick again - HOW do I get it unbricked?

Technical Support for Routers
4

Have you tried following this post

This file

https://github.com/gl-inet/uboot-for-qca95xx/raw/master/bin/uboot-gl-ar750s-20201222-md5-901bf6ffffc79ff06aec709b669fdc18.bin

Should be what you need. Flash this in u-boot then flash the most recent stable firmware from u-boot