That bike is not a mountain bike and is more of a lower end hybrid that is fine to ride but not worth putting a lot of money into and especially one that will change the geometry of the bike and is also not designed for the bike. The bike is probably OK for light duty off road stuff but not for mountain biking and putting a mountain bike fork on it would not be a good idea. Also you probably don't have much clearance for proper MTB tires on the bike.
If you are stuck on Marin here are some actual entry level mountain bikes with air shocks:
https://www.marinbikes.com/bikes/2023-team-marin-1
https://www.marinbikes.com/bikes/2023-team-marin-2
https://www.marinbikes.com/bikes/2022-san-quentin-2
https://www.marinbikes.com/bikes/2022-pine-mountain-1
https://www.marinbikes.com/bikes/2022-pine-mountain-2
And some entry level full suspension stuff:
https://www.marinbikes.com/bikes/2022-rift-zone-29-1
https://www.marinbikes.com/bikes/2022-rift-zone-29-2
Otherwise there are tons of awesome bikes for mountain biking and generally you are looking to spend $1200 and up for something decent with air shocks that is a capable mountain bike. There are certainly a load of bike listed as mountain bikes but in many cases the parts are not super reliable and generally you miss out on air forks and such.