Is GL.iNet Flint 2 (or 3) better than an ISP router/modem or most Asus/TP-Link routers for privacy/anonymity/security?

Far better than ISP one. Actually ISP versions usually backdoored. And you should not trust ISP at hardware level as it opens new possibilities of surveillance and even attacks.

But regarding other ones, if you cnow how to and wish to flash OpenWRT there is no big difference. If we are talking about stock router (eg TP-Link) firmware, yes Gl.iNet is far better in functionality and security (regular updates).

TLDR: If you flash OpenWRT - no big difference, if not - go with Gl.iNet