Not to get too deep into religion... but I'm Jewish, and in Judaism we have the Twelve Tribes of Israel. Which is explained, theologically, as the Israelites being descended from each of the twelve sons of Jacob, aka Israel.
These patriarchs of the Tribes are... well... not exactly fleshed out in scripture.
They're archetypes. Maybe based on real people, maybe amalgamations of different figures, maybe wholly fictional... but regardless, what is written about them primarily serves to explain and give context to each Tribe within the broader community of ancient Israelites.
All of this is to say that because of my own religious background the Thirteen don't really bother me because they fit a recognizable context. I can see what they're trying to do.
Stuff like them being the first of each different type of Transformer, or being short on fleshed out characterization, doesn't necessarily bother me because it fits with what I think was one of the inspirations.
These aren't necessarily supposed to be fleshed out and well rounded characters. They're mythical archetypes and patriarchs (and matriarch in Solus' case) whose exploits are supposed to be the realm of the deepest, furthest myths and legends.
Are they supposed to be actual semi-divine figures? Normal Cybertronians whose exploits were mythologized? Or wholly legendary with no historical basis? Well I'd argue just like IRL myths and legends that inspired the Thirteen... that's up for you to decide, and that's half the fun. Everyone comes at it with their own perspective. I myself prefer a more Euhemeristic take, so IDW1 intrigued me until Barber made everything Shockwave's fault.
I guess what I'm saying is that large chunks of the Thirteen not having many fictional appearances to flesh them out doesn't bother me. Being well rounded characters wasn't the point, strictly speaking. The whole concept is mirroring IRL pantheons and founding myths that treat founding figures as archetypes.