I’m a self-confessed Nicolai fanboy but I’d never buy an Argon of any varierty, but not least the Argon FR (I know you don’t specify the FR, but that’s the one I test rode).
They are just too damn uncomfortable beyond about 90 minutes riding. They are bullet stiff but while they fire you forward like a high velocity round, they also impact your body like one.
It’s a free country and everyone is free to choose whatever bike they want, but I wouldn’t go with an aluminium hard tail because they do tend to be harsh. As a younger man I rode one, but I am no longer in my 20s!
If you’ve got that kind of budget then personally I’d be looking at either titanium (something like the Ragley Ti, which coincidentally I just bought or One one Ti456, both of which can be used with forks from 100mm to 150mm) or a custom steel hardtail (which I investigated also).
If you’re going custom steel, then either Curtis or 18Bike in the Peak District, can sort you out.
The Curtis could be built to your needs for about £850 and would be around the 5lbs mark in terms of weight (Reynolds 853 used for the front and 725 for the rear IIRC), but it would tailored, not bespoke so you could control general geometry, but not the finer detail.
18Bikes can build you a completely bespoke bike; everything from the geometry, tubes, style of the stays, cable routing, everything. But it would cost at least £1000 and maybe as much as £1300 if you started adding things like tapered head tubes, custom drop outs (which I think can allow you to run SS and belt drive) etc.