Cmrls23, I am 70kg and have been foiling for 5 years. Have acquired a lot of different Axis gear. I have lots of experience but not a lot of talent.

Can gybe dead easy, wave ride a bit and tack inconsistently.
I started on the BSC 1060 with a 500 rear and windsurf fuse. Super stable platform. Great to learn with. Put it on waves and it struggled to keep up - too slow and draggy. Also found it a real chore when you did try and ride a wave. Heaps of lift, and then as you run down the face the lift lessens and you and up having to quickly change your weighting to not plummet the nose into the trough. You are right to move to a black fuse compatible foil.
Your choice of a short black fuse and 400 p tail I think is a good one. It will be a lot less pitch stable than the red fuse setup you've been using but it'll be much better on a wave. You'll keep up easier and will find the lift control much better to deal with.
I've messed around heaps with the png 910 and love it for all round. It's got a great bottom end, is predictable and manouverable and copes pretty well with waves. If you were conservative about where you go next, this would be a dead set winner. HOWEVER, you may find you hit your limit with it sooner than the spitfires or arts.
I've used an HPS 1050 and found it to be OK. I've used an HPS 930 and it was good. Not the bottom end of the png910, but much better on waves and for speed.
I also have a spitfire 840 and 960. These things are slightly more difficult to use than the HPS but feel a bit faster and manouverable too. The 960 stall speed was impressively low as well.
I have an art 999 and i've found this to have a bit more glide than the SF 960 or 840, but it's harder to use.
Going forwards for you:
What's your budget like? Theres been lots of HPS come up second hand for pretty cheap. These are a good deal as you get a good foil for a good price. HPS 980 would be a good one. I found the 930 would get up and running in 13-14 knots, so I reckon in 15 knots, the 930 would still be a goer. And they have a great top end.
If you decide to go brand spanking new, then I'd recommend the Spitfire. As for size of SF, well, I've found the 960 gets going in a little less than the HPS 930, perhaps 11-12 knots? It has a great top end and just glides down waves and is really predictable. While I haven't ridden a SF900, I've think they may be a touch harder to get going, with a little better top end and manouverability. The SF 840 needs close to 15 knots for me to get it to fly. It's an absolute scream on a wave though.
If you're worried that the whole setup that you might get may be a handful, then get a 450p. I find these to be a little slower than the 400p's, but definitely give you a more stable platform.
If you stick with the 400p, and get a black front foil and struggle, bung the 500 rear on for a session or 3. It'll anchor it and make it easier to play around on, but the foil won't glide like it's designed to. It'll provide a stepping stone to the more advanced setup. This is a way of progressing to your more advanced setup.
Good luck with it. Am sure you'll do well.