Dendrite doesn’t support sliding sync afaik https://github.com/element-hq/dendrite/issues/3236

Other than Synapse, I know Conduwuit has sliding sync support.

Neat! Glad to hear
Yeah they’ve done a good job with their documentation.
I was tempted by it several times already to try some features I didn’t even consider using.

Yup.
Tailscale has some documentation about it, and also a bunch of examples (And apparently one specific to Immich).

Since you already got some replies, a bit of a different approach would be to set those services up using Docker and having Tailscale as a sidecar to each one of those.
You will then be able to access each one as a seperate device. immich.*.ts.net etc.