Migrating from UNet to TNet wasn't too difficult if I recall correctly. However, I wasn't using 3rd party assets, so I was very familiar with all the code.
[RFC] replaces [ClientRpc] and [Command]. To make it execute on the host (like the Command attribute) you simply check TNManager.isHosting.
[SyncVar] doesn't exist, but you can use tno.Send with the Target.AllSaved to achieve the same functionality. Except you have more control now because you control how often it's synced.
I'm sure there's more that I'm forgetting, but in the end I think you'll find TNet comes with much less hair pulling than UNet. If you do decide to make the switch, be sure to check out the tutorials to familiarize yourself.
Oh, one important thing: TNet isn't server-authoritative. The host has authority. You could probably fake server authority by making the host a dummy, headless client, but I felt it's important to mention that. I don't think you'll find a network solution that's actually server-authoritative. It's (probably?) impossible to make a one-size-fits-all server-authoritative solution, as it's very game-dependent. Regardless, though, you can absolutely rewrite TNet to be server-authoritative - Aren has the most wonderful coding style I've ever seen - as everything is very extensible and simple to customize, but it is a big undertaking nonetheless.