IMO, I wouldn't do the Rock Hill Crits unless you are confident riding in a group and willing to push the pace. Just riding around is dangerous to you and everyone else. I don't want to discourage you, but I would skip this until you know what you are getting into.
55k is a whole different beast, if you want to be tested mentally, this is the race. The hike a bike sections and pure tech here will push you to your limit.
Here is my calendar of things I'd like to do this year, I'm sure I won't do all of them though:
I'll do all the Winter Short Track races as long as it isn't raining
Jan 19 - Whole Enchilada All the trails at WWC, I like that it's in the winter. Was run okay last year as it's first time, so hopefully they do better this go around.
~End March - Love Valley Roubaix Great gravel race, Would be good training to do on your MTB as well. Lots of climbing.
~Beg April - 6 Hours Warrior Creek Another good race that you can show up and have fun at. The trails are super fun and you could do this as a team instead if you wanted to. Great vibe, a must do, and I hate lap races!
April 20 - Bootlegger 60mile there is a 100 mile version, but I'll stick to the shorter one me thinks. Unless someone peer pressures me into doing the 100. I have only heard good things about this gravel race, and again, would be great training to do on your MTB for the long climbs.
~End April - Saluda Roubaix 68 Another fast gravel race that is sort of local, not as much climbing and the speeds seemed pretty fast. Was close to my birthday last weekend, so that was the excuse to do it
~May 12 - Rock Hill Double Down Crit 50/50 on this, but I'll put it on the calendar. Seems like a fun concept.
May 19 - Pisgah 111k Rumors it may be a NUE race for 2019? Maybe someone can chime in. Since the rain 55k course was on most of these trails, I really wanted to graduate from the 55k to this race. Doing this only to say I did it, I think. The fast dudes did this in 6:30, the humans did it in 8+ hours, so a real mental test for sure.
June 15 - Tour De Kale Not a race, but a really well organized cookie ride. It was so well run that I will probably do it every year. Water handouts, police escort, cold towels at the top of the final climb, food at the rest stops, food at the end. As close to a circuit road race at the pointy end than anything.
~Beg August - Crossroads Classic That whole 3 day weekend intrigues me. IF I get a USAC license for 2019 maybe I give this a whirl. 2 Crits, 1 Circuit race.
~Beg Sept - 6 Hour Treeshaker Anne Springs was a blast at JJFit Fest, bummer they didn't do it again this year. But this seems to be the exact same thing, with another 2 hours? Sign me up.
Hope that helps!