This will last for about one month, then you will be hooked on road bikes,
this for me. Got a £1K C2W bike after 20+years away from the road, 18 months later bought a plastic fantastic with full ultegra. Ridden it more (either time or miles) than my ~£4K Alpine 160 and Yamaha R1 combined.
Go by top tube length not seat tube. From borrowing a bike I figured 550mm TT was right for me (5’10” fairly normal proportions); I borrowed a 55cm ST Bianchi, bought a 54cm ST Orbea and then a 52cm ST Lapierre.
I think you need to get in to road riding.
my very first ride on the borrowed bianchi was a hateful deathmarch and I wrote a vitriolic post on our club page about being a mountain biker not a cyclist! Since then it “clicked”, and despite doing a few long weekend road trips riding the best MTBing that Wales has to offer on a bike I love, my single best day on a bike this year was the Ronde Van Calderdale sportif, riding 25%+ gradient yorkshire cobbles with deep snow on the verges.