Long story short, I was going to trade in my 2017 Specialized Sequoia for a All City Cosmic Stallion. I estimate I would get $700 for the trade leaving $2000 difference for the complete bike with mechanical post mount brakes.

Option 2 is to buy a Gunnar frame and fork and for now take as many parts off the Sequoia to make a decent ride until I upgrade to ultegra.

Seems most of the parts on my Sequoia will fit on the Gunnar Hyper-X.
Will many of the parts work on the new frame until I upgrade?

Everything should be completely fine, even the bottom brackets are the same, BSA 68/1.37/24 tpi. Things haven't changed much since 2017. But why? I don't know much about this class of bikes, but is one that much better than the other?

My Sequoia is a very heavy bike frame, also prone to rusting.
The Gunnar frame is a better quality frame that should last me a lifetime. Starting from base frame will be easy to treat with frame saver.

I also have always wanted to build up a frame, have most if not all the tools needed to do this.