I honestly am in the same boat regarding Go. I played for a few weeks and that was it. Considering I don't have any other Nintendo hardware outside of the switch, the last pokemon games I played were silver and gold (aside from playing soul silver and heart gold on the emulator). Actually, I think I played Emerald or w/e it was at one point. Maybe it is the nostalgia hit but it looks interesting to me.
I have been thinking about getting a 3DS to get Ultra Sun/Moon. Considering the above, how far behind am I in the franchise and is it even worth me picking up at this point?
The franchise rarely ever requires you to have expansive knowledge of the previous generations. Aside from the introduction of new pokemon every generation, there's very little that's changed the games so drastically that you'd be lost if you jump in now. The biggest changes would be the Physical/Special split from Generation IV (moves are no longer classed as physical attacks or special attacks based on type. It's now unique to each move) and the addition of a few new battle types (double/triple/rotation battles), as well as the creation of the Fairy pokemon type. The first one is pretty intuitive to grasp once you see how it works, the battles are either explained to you when you first encounter them or are simple to understand the altered mechanics of, and the Fairy typing may cause a little confusion figuring out type advantages and some of the reclassified pokemon but isn't really a big deal.
Various newer games tend to have quirky fancy new mechanics like Mega Evolution or Z-Moves or whatever, but those tend to only be prominent in the games they were introduced in and then not really harped on afterwards and will usually be explained within the game if they have some level of importance.
Your best bet might be to start with OmegaRuby or AlphaSapphire. They're 3DS remakes of (shockingly) Ruby and Sapphire from the GBA era. They'll be a nice balance of leading you into the newer game setup with stuff that will be similar to where you left off but also feature some of the newer mechanics and ideas. From there you could either move onto X/Y (which I believe were designed to be good entry points for newer fans) or right to Sun/Moon or UltraSun/UltraMoon.
It is worth noting that Sun/Moon and their Ultra counterparts are known to be a little more story-focused and "cinematic" than the previous games. Lots of time is spent watching the story underpinning hte adventure unfold and the game does sometimes feel like it's railroading you into continuing that adventure rather than simply letting you wander about the land trying to be a Pokemon master. So you might find that aspect of the game off-putting if you're used to the way the earlier games guide you on a path but rarely ever feel like they're shoving you to constantly keep moving.