Great theory, Jofwu. I've been thinking similar things recently, and I like the way you laid it out here.
I'm not sure what other sentient spren we know of that aren't Radiant spren. Furthermore, I think the spren in the fabrial must at least be related to the Radiant spren, as presumably the spren for any individual fabrial would be related to the order of KR that can access that Surge. I don't think an honorspren in a fabrial would give you an Oathgate, for example. I don't know that any of the Radiant spren would want to be trapped in a fabrial forever, but I'm not sure how else you could access surges. The only other thing I can think is, if there are Surgespren (like gravitationspren, adhesionspren, etc), you could use those instead.
The fabrial spren being KR spren may also help us to understand why some fabrials use multiple spren, as you suggest might be the case for some Soulcasters and we know to be the case with the Oathgates. We know now that the different orders can access the Surges in different ways, e.g. Dalinar can use Adhesion differently than Kaladin (might be getting the specific Surge wrong here). Perhaps having multiple different spren allows the fabrial to access the Surge in all possible ways? This may explain the limiting factor on the Soulcasters, though we don't know much about the difference between Lightweaver and Elsecaller Soulcasting. I'll toss out another idea here related to the Oathgates. When Jasnah used the Transportation Surge to escape the ship in WoR, it looked like she ended up in Shadesmar in a corresponding location to where she was in the Physical Realm. I think it's possible that the Willshaper use of Transportation involves moving around within Shadesmar, and the combination of these two effects in the Oathgate (through the two spren) allows you to travel quickly "through the Physical Realm". One spren pops you into Shadesmar, the other transports you to another Oathgate in Shadesmar, and then the spren at that gate pops you back into the Physical Realm.
Many possible questions to ask Brandon here. Knowing more about the differences between the ways the orders access the surges would be a huge help.