Right now the biggest battle you're going to have is the lead times. With everything that's going on nothing seems to be staying on the shelf per se'.
With that said; here's my 2 cents:
I've used the CK Arms frames for a few of my builds. I believe The 'C' is Matt Cheely if I'm not mistaken. The frames are very nice and there's a couple details that are added that you wouldn't really know about until you built the gun. I have also built using the Cheely frame and as expected the quality is top notch.
Personally I would stay away from the STI frames. My personnel open gun I built on an STI frame (it was what was readily available at the time) and it's fine, but from the builders perspective I have to do more 'massaging' to get it the way I wanted it to be. (a lot of what I an speaking to is form and fit type work)
I am going to start a build on a Limcat frame in the near future (owners choice), however I can't speak to the quality as this point, but going by the open guns that I have worked on, they are very nice. We'll see when it comes down to fitting etc.
Others that are out there but I have never used:
Phoenix Trinity (I'm pretty sure Altas used to use their frames)
The only 80% That I have every worked with was from Limited-10; who are now Rogue tactical. The dimensions seem to be within spec, however I did get a frame that is out of spec a bit and causing me grief fitting a steel grip. It's a personnel build so I'm not sweating it. Overall experience with L10 (before Rogue-No experience yet with them) was positive and easy. I would most likely order from them again. I'm almost positive they make them in-house and made to order. (I'm pretty sure I am going to order from them again soon)
Currently I am waiting on a couple of frames from another company out of Arizona. I will say this: I ordered them on November 4th .. still not shipped and I am super frustrated. Communication has been so so and I never get a real answer; I'm just told soon. as of this writing they did send me a tracking number, however it's not even at FedEx to be shipped. So I have no idea what's going on. I left the name out because I don't want to start bad mouthing a company just yet. Times are pretty messed up, I get it, so I am being a little more forgiving these days.
There is a company out there called Optimum manufacturing. It looks like they are job shop and have their own line of products. There stuff looks pretty good, but I don't know if they have a minimum quantity order. I'll probably reach out to them soon as they really peaked my interest.
I can go on and on in regards to the parts etc. I will say this: You will enjoy building if you like to tinker etc. Have patience! Measure, measure and measure. It looks easy to build; but there are a lot of details that you need to pay attention to. Building and assembling are two completely different animals; don't confuse the two.
Always use quality parts and you will have no regrets. There are tools you will need, but it sounds like you may have them already. If you go the 80% route you will need a mill, I guess that's obvious, however Matrix Precision sells a jig of sorts to cut the rails and drill I think the sear and hammer pin holes. To get a good look at this jig in action checkout Mosin Virus on YouTube. He has a nice series where he uses these jigs and walks you through a build he is doing.
I hope this helps. I'm sure more seasoned builders than me will chime in. Good luck with the build. fell free to ask questions if need be. I'll do my best to answer them.