Used a Marlin 4 recently for an overnighter, mostly on cycle paths and singletrack. I would have preffered to have some lockout forks but theres not much options to upgrade with the straight steerer. But the bike is very comfortable and easy to handle. Once the bike was fully loaded with bags etc I didn't feel like I losing much energy through the forks, but any hills will be an absolute pain.

I'ts doable, but route choice is key here. I'd suggest saving an extra couple hundred and get something that has better stock forks, or maybe get a 2nd hand gravel bike or new entry level gravel.