The whole thing about mast causing hair loss is not entirely accurate - that can happen, but can happen with basically any AAS, since all of them will cause increase DHT conversion which can lead to hair loss. Mostly it depends on your genetics. I had some thinning on mast, but no full-on retreat of the hairline. But then male pattern baldness does not run in my family.

Have you run your bloodwork? Before you do anything else, find out where your estradiol is on your current test dose. Once you have that number, what I would suggest is keeping test relatively low and put in the mast at 150-200mg/week. See how you respond. Run bloodwork again 8 weeks in. Don't implement an AI preemptively. There's no need unless you start getting itchy nips/signs of gyno and bloodwork confirms high E2.

Then, when you have the test/mast baseline established, add in the NPP - that's a short ester so you are committing to at least EOD pinning. I would keep it moderate at first: I started with 10mg/day and worked my way up 35mg/day, pinning every day. NPP takes about 10 days to reach saturation, so I would schedule titration up to 200-300mg on a two-week basis so that you aren't getting in front of yourself. By two weeks on a dose you will have a good sense of how you are responding to the NPP, and whether or not you should increase. Find the sweet spot - lowest dose for the most gain - and hold there with the mast and test steady. You should see some solid dry gains.

Cruising at 300mg test E seems high. My total test sits around 900-1000 when I cruise on a mere 125mg/week, and that's more than sufficient to maintain and even increase gains.