It could be trim or C of G, or a combination of both. In a perfect world both elevons would be lined up with the trailing edge of the wing and the plane would fly level at your cruise power setting.
But the world isn’t perfect, and it’s very common for flying wings to fly best with a couple of mm of upward deflection in both elevons. Wings are notoriously sensitive to c of g, and if you go too far aft the plane will not be fly. As was said earlier, the FC will mask a lot of that until it’s too late. If you’re going to adjust the c of g of the plane aft beyond what the manufacture says, do so with caution if you aren’t sure what you’re doing. The marks the manufacture puts on the wing are generalizations, and they will bias forward because it’s safer, so it is common to adjust aft of the limits, just be careful. The plane is flying now, so make your adjustments a few mm at a time.
This is some really general advice. Setting up trim and c of g is hard to describe without being there.
I always set SERVO_AUTO_TRIM = 1. This will automatically trim the controls in flight. You should start with the servo trims at 1500 and the elevons should be physically lined up with the trailing edge of the wing. After your flight look at the new trim settings for your elevons. If the trims are just a small bit out, then you can adjust the pushrods to Center the servos. If the trims are really far out then you should move the c of g a small amount and try again.
How much is a small bit or too far? It’s subjective. For me, if the trim values are within 50 (1450 to 1550) then I’ll often I’ll just leave it and won’t mess with it unless I’m in the mood to tinker more. Within 100 (1400-1600) then I’ll probably just adjust the rod ends to centre the trim. More than 100 and I’m going to start to look at what else could be the cause and that’s when C of G will come into adjustment.
Do not use the trims on your radio!! That’s for traditional RC planes and will only cause problems with flight controllers. Your radio trims must be set to zero. If you can, you should disable the trims in your radio so they don’t get accidentally adjusted.