I suggest you ride it stock for a while and see what doesn't fit.
I concur except for more lights. Nobody seems to be able to see a bike (2 wheeled or 3). The more lights you have, the better off you are. I prefer a tasteful lighting upgrade but I've seen some National Lampoons Christmas lights, too. I think those look funny but at least they'll be seen.
Get a bluetooth headset for your helmet(s). There's nothing like having the sound right in your ears. AND, you won't be deafening people around you at stop lights. This makes you a better ambassador for the Spyder community.
I fully agree on this one. I happen to have multiple bluetooth headsets for multiple lids (although I tend to use just 1 of each) but even if I like your taste in music, I don't want to hear it. Could very well be nobody else wants to hear it, either. I had some pizza delivery guy pull up in my driveway yesterday with his music LOUD. I told him to turn it down or leave. He left, I was hungry.
For 2021, BRP has handlebars that are adjustable, but only the height can be changed.
This is an added cost option. It's an adjustable height riser.
ANY F3 handlebars will fit the 2020+ RT (regardless of trim level). The F3 "short reach" bars will bring the bars closer to you. As Three Wheels said, Helibars allow you to adjust the bars in multiple areas but they're about $600. (I got 'em and love 'em.)
You should also consider a drivers backrest for lumbar support when riding over the long haul.
I suggest Utopia but there are other, less expensive, options.
Many people buy an upgraded sway bar, but in my opinion, BRP finally, at long, long last has caught up to a lot of the suspension upgrades. So, again in my opinion, the 2021 Spyder handles far better than all the previous models. A sway bar upgrade makes the Spyder handle "a little more" far better.
I added a BajaRon sway bar (nothing better and he's a site sponsor). In my humble opinion, it's well worth the money but doesn't improve the 2020+ RT as much as the previous generation RT. Still glad I installed it.
The radio, speakers and bluetooth options have all been upgraded and included in the new model. The electronics are pretty good, but many, including myself don't like using a smartphone connected to the dashboard for navigation.
The radio, speakers, BT, etc have been the same since '18. The phone/dash thing is called BuRP Connect and it's NOT ready for prime time. Quite honestly, it sucks. As others have done, I use my phone directly BT'ed to the bike for music only and have a dedicated GPS for navigation.
After about 6 or 8 thousand miles, you may find that the rear tire is wearing out. That might be a point to upgrade to better tires. There's lots of threads on tires for advice.
Car tires, car tires, car tires. The current favorite is the Q'stein F5 (V5? C5? someone will correct me). When my current Kendas wear out, that's what I'll put on all 3 postions.