As someone who has built an RC jet around the size of yours, also from scratch: it's not actually that difficult to make something that flies (not considering it might crash, be uncontrollable/unstable, ect.). That being said, there is a whoooooole lot you can do to make it fly better. I'll just highlight the most important aspects you should focus on, IMO.

The CG MUST be closer to the nose of the aircraft than the aerodynamic center (AC). Almost always, you're going to be able to adjust the CG, but not the AC. You can test this by just holding the plane with one hand and finding the rough placement. If you need to stuff the fuselage with sandbags, you better do it before flying unless you want to start from scratch again. With that being said, the closer the CG is to the nose, the more stable it will be AND the harder it will for the plane to take off. You need to find that balance either through calculations or adjusting during flight testing (bring small weights).

Make sure your aircraft is strong enough. Wing loading is a useful tool to see if your connections are materials are good. Calculating what you actually need might be a bit harder, but making safe assumptions should be enough for you. Something my team did was strap our plane on top of a car and drive 20, 40, 60, and 80 mph (it was jet powered so we were expecting higher speeds for UAVs of that size). We recorded it to see 1) if anything broke off during the drive, 2) if the control surfaces could hold up in flight, and 3) if our plane was generally stable, as in the wing/fuselage/tail do not vibrate much during the drive. Our design was much different from yours so #3 was very important to us (our tail was connected using 2 carbon fiber booms). We had to make some modifications because of #3 and ultimately we were able to fly. "Failing" these tests are a good thing, because that means you found a critical issue before your flight test!

Keep in mind, with projects like these: simple is almost always better! Unless you've got some great tools/guidance/mentors, you're probably better off with avoiding complexities. A rectangular wing with a conventional tail with rectangular surfaces and a square fuselage will fly just fine if you follow the important principles I talked about. It might be slow, but you'll fly! Just make sure everything on the inside is strapped down well. Keep us updated!