I'm trying to figure out using my OpenWrt router as a Tailscale client myself and am still trying to get it to work properly, but for the memory issue I can help you here. I had this same problem on my router but you can fix this by getting a USB drive and installing xroot on your OpenWrt router. Make sure you use a reliable brand like SanDisk or something. This is the video that helped me do this: https://www.youtube.com/watch?v=5UjCT99udm0
Also make sure before you run the install part that the thumb drive is actually plugged in as stupid as that sounds I made the mistake of thinking you had to do the install before hand and that made me need to restore my OpenWrt to defaults lol.