I’m sorry no idea about tailscale specifics but you find details in Collabora integration guide
I assume the problem as you analysed already - likely some connections are not possible e.g.
start checking connectivity using curl or wget as described in the above guide. additionally you configure
which is CODE URL - aliasgroups is supposed to hold Nextcloud URL..
as 3rd topic I see http:// URL everywhere. WOPI protocol requires TLS - maybe there is another problem.
There are few topics related to tailscale - look if you find useful hints especially Local access for Collabora is focused on CODE, maybe AiO guide is helpful as well Tailscale (and Caddy as a sidecar) Reverse Proxy · nextcloud/all-in-one · Discussion #5439 · GitHub