It depends. If you buy everything now and the motherboard is unavailable, you have a collection of parts you can’t use for anything until the motherboard is available. That doesn’t sound like fun to me, but if that’s what you want…?
If you see parts like the RAM, SSD and USB-Ethernet adapter available now for a good price, GET THEM. The way parts availability and pricing are these days, if you find something you want, get it immediately. There’s no telling if it will go out of stock, if it will ever go back in stock, or if the price will increase tomorrow.
Here in Canada, Framework’s pricing on all 3rd party components was more than fair. I only ended up getting the SSD myself, and that only saved something like $20, and that’s since been corrected.
You may want to check with this forum regarding 3rd party RAM. The Framework laptop uses DDR4-3200. It must be native DDR4-3200 and must be rated for 3200 without using XMP. There’s some “DDR4-3200” out there which is really DDR4-2400 or DDR4-2666 which only runs at 3200 MHz using XMP - and the Framework laptop doesn’t use XMP, so it would work but at 2400 or at 2666.
The pricing for Framework components here seems to be fairly stable, so get them whenever you want. This is not true of 3rd party components though.
If your goal is to save money, you should just get a barebones DIY for 1050 USD. If your goal is to avoid buying components you don’t need (e.g. to reduce waste) then, barring availability, I also don’t see a reason why you can’t do what you’re saying.
From your shopping list it seems like the only difference from the DIY is the lack of battery, which should be no issue provided you update the BIOS to v3.07. Also, you won’t have the awesome screwdriver