The Quick will lose the length of the bars---probably 70-100 mm--off the total reach if you perform the conversion. I can't find the geometry charts for the two frames (honestly didn't try hard---they aren't immediately obvious on the C'dale site so whatever) but you'd need to figure what the new numbers would be for the converted Quick and see if it would fit ....
MUCH easier, especially since you admittedly aren't too up on all the bike stuff, to get a new bike. Buying the right parts and paying for the swap could get pricey. And then, possibly you wouldn't like the new build anyway.
If you want a lightweight flatbar gravel bike, get one. I am not usually in favor of N-1, but in this case, if you know exactly what you want, get it. The two older bikes look to be in decent shape so you might get a decent price for them.
If it were me, because I have all the tools and like fiddling, I might go search for the geometry charts and figure out if I could do the merger .... I'd slap the old and leftover parts on some other frame maybe, just for kicks (but I actually have no more room in my garage .... hmmmm....)
If you combine the two bikes, you would be left with parts and a frame which didn't work for you and only one ridable bike anyway .... and possibly that one bike still wouldn't fit right.
Sell the ones which are not what you want and buy what you want.