Just picked up a Highball to run SS. All I can tell you so far is that the frame is over 2kg so it won’t be XC Race light. Rigid SS will be around 19lbs which is heavier than my Chinese 29er with gears, suspension and a dropper.
If it were me I would buy a Scale although the complete bikes aren’t great value as you say. Radon are really worth a look too if the availability is good.
Your Zealous with a shorter fork won’t be that far off modern XC bikes like the Scale and F-Si. The short chainstay is the glaring difference and the BB may end up a touch low but the sizing and angles will be much the same. Perhaps worth looking in to if you decide the XC racing thing isn’t for you in the end?
Another thought… Is it an option to spend the same money on frame, forks, wheels and tyres and reuse your current components? SIDs for ~£350, 1.4kg wheels for ~£500, tyres ~£50, then whatever takes your fancy on the frame? I went for a 256-sl from Hongfu (almost identical geometry as a Scale), 1kg frame, and about £400 all in with ~2 weeks delivery
EDIT: In fairness the Trek in your link is indeed looking pretty good 😀