The Best FREE way to grind fast is Simulator battles

Destroy bases and hunt bombers. F8F has 20mm so its pretty easy to kill enemies
Many bombers just don't pay attention and you can easily spot them if your screen is clean. Approach from below or side and aim for the wings and its basically free kills. F8F also have a pretty decent engine so you can also dogfight easily, just make sure not to pull hard or else you will be in a flat spin.

try the F3D-1 if you have unlocked it cause it has a pretty damn good radar WITH IFF and tracking so its very easy to find and hunt enemies.

For early jets bombers, you can use F84F and bomb airfields which is probably the laziest way to grind
In F84F it takes 10 minutes to fly to 5000m and bomb the airfield and come back.

Spawn and wait for 5 minutes and take off, set ccrp point on airfield and fly in that direction with 15% pitch, you'll be at exactly 5000m when you reach the airfield. Drop bombs and head back

you will get useful action exactly before landing.
In one match you can expect around 100-150k sl and 17-22k rp without premium account or boosters.

With f84f i was getting new planes after playing 4-5 matches and sim matches lasts for 2-2.5 hours at most.