I’ll get the controversy out the way first - in my view Gravel was borne from the bike industry in an attempt to sell us something we don’t need. And it’s a euphemysm for “this ride would have been more fun on a MTB”!! (tongue firmly in cheek!!)
There’s nothing wrong with the Marlin. It’s a good entry level bike. Main down points are the QR’s, the forks and the weight. I personally wouldn’t drop that amount of money upgrading the Marlin as you’ll reach the point of diminishing returns pretty quickly. Especially if you have been looking at MTB’s in the $1500 - $2000 range. However…
If you were to stretch the budget by around $200, you could be looking at a Trek Procaliber 9.5. Stock, that bike is 11.64kg (25.66lbs). You wouldn’t need to upgrade anything on it at all, unless you wanted to. But, you’d have a really great platform for upgrades if you wanted to in the future. It’s got a nice slack head angle for a XC bike so it handles superbly. It’s a brilliantly fun bike. I’ve ridden the same trails on this hardtail with 2-pots and 100mm of front travel on the same trails I take my Fuel Ex on, and have just as much fun!
I got luck and found a used 2021 model for £995, so part exchanged my TT bike for it. I spent the change on some performance/weight saving upgrades, which has taken the weight down to 10Kg. Eventually, it’ll get down to a sub-9kg race bike.
Demand is high for used bikes, so you’d probably be able to sell the Marlin and stretch the budget on a new bike.
A word of advice, which I learned the hard way - you’re better off with a newer bike with modern geometry but lesser spec, than an older bike with outdated geometry and high spec!!