First, sounds like you are making some good progress transitioning from the UltraCat (UC)to the Futura 144 (F144).
First, to enable you to tack more easily on the F144, let me remind you that short board tacks must be made more quickly than on the UC.
The Futura has most of it's volume between the mast foot and the front footstraps.
The UC has lots of volume forward to the mast track.
So, you cannot "walk forward" around the mast like you can on the UC as there is smply not much "float" (volume) forward of the mast slot on the F144.
I would suggest a little "modification" of your technique in the near term, and you may want to keep it in your repertoire for future use.
Instead of stepping forward of the mast foot, simply "step over' the mast (with the sail raked all the way back until the foot rests on the deck of your board.
It's easy...... simply step over and turn around as you bring your new front over the mast.
This will keep you in the center of the board (side to side or athwartships) and keep you in the highest volume are fore and aft.
A waist harness and PFD can indeed be a problem. I had to shop around quite a bit to find a kayaking vest (Perception Kayaks makes the one I use) that did not interfere with my seat harness (a waist harness simply rides too high to use with almost any sort of PFD).
The kayak vests are short so they can be worn over the spray skirt on a kayak, and this makes them ideal for use with a seat harness and a low harness hook position.
Jibing the F144 is going to be alot different than the UC.
First, you cannot walk way back on the F144 like you could on the UC. There's not much volume behind the front footstraps, so you will need to tip/bank the F144 so it turns downwind and back up to a beam reach (or lower) on the new tack, and flip your sail when you are heading very nearly straight downwind.
So, if you were jibing you UC with the centerboard down, and had to tip the board to the outside of the jibe to get the board to come around, you will have to tip it the opposite way (bank the board into the jibe with the inside rail down).
8.5 m2 is a little big learning things on a smaller board in 15 mph (13 knots) but it sounds
like you were not having any issues with the sail size, so go ahead with that 8.5.
You might find that learning "sail handling in the transitions (tacks and jibe) is a bit easier with a smaller rig.
You won't be able to get into the footstraps on the F144 until you are fully planing, so it's best not to try until you find yourself back far enough on the board to get your front foot into the footstrap.
You also cannot really get the F144 to plane if you continue to stand up forward by the mast foot. The rockerline in the board is designed so that you almost have to get your weight back with your rear foot about midway between the front and rear footstraps before the nose of the board (and the rockerline) come up so the board wants to "slide" up onto a plane.
If you stay forward (ahead of the front footstraps with your weight, th eplaning surfaces on the bottom of the F144 will remain "negative" (ie lower in the front of the surface than at the back of the surface).
So, with negative planing surfaces, the board will pretty much refuse to jump onto a plane.
This was not the case with the UC with it's huge long waterline and nearly flat rockerline.
Going from a longboard to a shortboard is not too difficult, but you need to make some major technique changes.
Hope this helps,