One question. Where would you get the Seekins parts? I haven't seen the IRMT upper and handguards available anywhere. I think you'd have to buy a Seekins DMR just to get the main receiver parts, which probably would be a good buy on it's own.
I don't think Proof barrels are worth the money anymore. For the money I would buy a Craddock RTR, a WOA, or just get a custom barrel made so you can run an intermediate gas on a 16" barrel. I think your challenge with a 16" 6 ARC barrel is getting the right gas length. Mid length is the common answer on 16" barrels but on a 6ARC I would guess you'd definitely want an intermediate length at a minimum, especially if suppressed. I am also starting to develop the idea that a person can buy a mid-tier semi budget barrel for a bolt gun with a fairly reasonable degree of confidence. But in a gas gun there are more variables and opportunities to bite you in the ass with the gas port and barrel extension so that the risk you incur with a mid tier budget barrel is much greater. The way some barrels begin to catch copper on the gas ports and erode quickly is a little bit of a mystery to me and the only way I know to mitigate the risk of getting a barrel like that is to buy from a custom barrel builder so you have someone that's going to back their work. Like Craddock.
I'm sorry, I don't have a Seekins small frame to compare to answer your main question but I do have an LPR. I really like it but after shooting it, pulling it apart and replacing components in it, I think it's over priced. If you make a planning assumption that the Aero Enhanced upper, Seekins IRMT upper, and LPR all have very comparable rigidity the best buy is going to be building off an Aero enhanced upper and handguard but I wouldn't fault someone for wanting to pay a little extra and just buying a Seekins DMR. Euro Optic has 16" 223 Wylde DMRs in stock for $1800 and Scopelist has 18" 6 ARC DMR uppers in stock for $1300.
You can put other barrels in the LPR but the handguard is pretty slim and with an aftermarket heavy barrel you may not be able to fit MLOK accessories on it due to the bolts being to close to the barrel. Seems like a pretty small thing but if you want to run side weights back towards the receiver for competition you may run into issues. You can put a slimmer barrel in it, but then you lose weight and potentially barrel rigidity. It becomes a catch 22. The stock Ridgeline Proof barrel will accommodate MLOK along the side of the handguard.
It's really a shame that a person can't buy builder kits in any of the higher end semi-monolithic small frames. Kind of a weird market gap. It seems like if someone came out with a rifle based on their builder kit that demonstrated little to no handguard shift, had a few handguard length options, and fully ambi bolt release they would do very well.
