I found an informative post about a related issue that might be of some use to you. Sounds like DHCP or Network Manager may be rewriting your systems-resolved.conf.

https://joshrnoll.com/my-tailscale-dns-woes/

Have you tried deleting /etc/systemd/resolved.conf and restarting the service with systemctl restart systemd-resolved?

Did you undo the reverse path strict filtering your guide suggested?

net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.all.rp_filter = 1

Above is what the guide suggests to force reverse path strict filtering. Try setting as shown below:

net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.all.rp_filter = 0

According to the guide, “By default, these are set in /usr/lib/sysctl.d/50-default.conf”