This is my plan as well, start with the chassis and find a newer mainboard second-hand. I configured a 2TB SSD and 64 GB of LPCAMM2 RAM to see the price for the new 13 pro and it was doubled from about 1500 CAD to over 3200 CAD before tax. Upgrading the mainboard, when I have a perfectly functioning (and fast) 12th gen Intel i5, is out of the question for now.
If you ever decide to buy the whole new system, buy it without memory and storage from Framework and of course DIY instead of assembled. You can get them cheaper elsewhere. They aren’t Apple-level overpriced for memory but they are noticeably overpriced.
This advice is a bit more difficult with the current LPCAMM situation. In the US, besides Framework, I was only able to find a 32GB module from Adorama, seemingly a left over new stock Crucial module that I assume must be discontinued in Crucial-branded form, or eBay. But it is far cheaper than Framework’s price ($250) and seemingly still available.
(Interestingly, Adorama’s website says that it’s “trending.”)
You’ll save at least a little bit of pocket change buying your SSD elsewhere at the very least.