Piling on here, and totally agreeing with Stelios, but perhaps with a different time line in mind. How long have you been using the MacNewt / 533 / AVX combo? Personally, I'd stick with what you have right now, and work on your technique; his #1. Timeline to #2 may be longer than you are thinking.
Deep Sky AP requires a bit of patience and practice to get the most out of your equipment, otherwise you're looking at over-killing the solution to make up for it, and that gets really expensive, really fast. Instead, get your balance adjusted in a way that your AVX likes it (they each have their own personality); tweak and optimize PHD2's parameters. You might upgrade the guide scope and camera, but you're not totally out in the weeds with what you have, as long as it's properly attached. Get really good at polar alignment, to keep the AVX's Dec motor quiet. If possible, mount your equipment to keep it as low, balanced, and compact as possible. Remove anything that doesn't absolutely have to move, away from the stuff that does. Watch the cable dress. Fine tune the focus (certainly my challenge!). Understand proper exposure, and the proper use (and not) of filters. This is all hygiene. Go for more data (hours) to bring out the fine detail. It can be there.
I've spent (only) 4 years at this hobby, making upgrades and adjustments in equipment, yes, but mostly learning how to use what I have. I started with an 8" f/5 Newtonian on that AVX mount. I kept improving, but also kept getting advice to dump the AVX and get a better mount. I might do that some day, but it keeps surprising me as I've made other changes to the system and how I use it. I still have it, because I keep finding it's not the weakest link. I am.
After hitting this month's Challenge target (Markarian's Chain), I stumbled across a cute little galaxy that was just begging to be imaged. I imagine it's the sort of target you're going after. This is NGC 4535, the Lost Galaxy, and it's about the same size as the one Stelios imaged, also with 4hrs integration. My scope is longer in focal length than your MacNewt (not necessarily an advantage - it makes it much harder to control), the camera pixels are the same 3.76u as your 533, and it's all on top of that same AVX mount.
To be sure, I could spend "500 Lattes" on a better mount, that would probably reduce the star size a bit, and improve the detail in the galaxy, but the limit in Seeing is what it is (it's not great here), and I expect doing a better job at (re)focusing would go a long way to closing that gap. Doing that won't require me to spend money on anything, but just get better at using the tools that I already have. And all that skill will directly apply to any mount, scope, camera, and processing that I do in the future. Stelios is much more experienced at both imaging technique and processing than I am. Imagine what he could do with my (or your) equipment?
So I'll work on my focusing, THEN I'll consider upgrading the mount. Maybe. No promises... Maybe after a reducer / flattener first? Markarian's chain was just too big to image. That said, mosaics can be free, too, and that's another skill I don't yet have.