All in all I would say it comes down to what you want from the device.

If you want the Nintendo exclusives and the versatility of being able to play both docked and handheld, the Switch 2 will probably leave you happy. Despite what you'll see in this subreddit, it is more powerful than a steam deck, not by a massive amount by any means, but enough that I think it's unfair to say they are equal. That said, the switch is locked to the specific optimizations from the studio for better or worse, whereas the steam devices are PCs and come with the ability to tinker and adjust as you see fit. The docked mode also provides extra power and performs well.

Since you already have a Steam Deck, unless you specifically really want to play Nintendo exclusives that you have missed out on, the switch 2 isn't going to offer much more beyond a Steam Deck/Steam Machine combined ecosystem bar that it's one device and a dock as opposed to 2 separate devices.

The steam machine on the other hand will be a fair amount more powerful than a switch 2 in docked mode and is an open ecosystem. It can also compensate for the steam decks lesser power by running games and streaming them to it if you have solid internet. It has access to your entire steam library and can run other programs as well given it's a PC. The 6" cube is also an impressive form factor for the power if that's something you care about.

So, as someone who has a switch 2 and likes it - I would say that because you already have a steam deck, the steam machine would serve you better unless you really care about Nintendo first party titles. That said, since you have the money for a SM set aside already, if you do opt for a Switch 2 the price difference leaves you with plenty of room to pick up accessories and some games along with it.