You’ll get many opinions. The rule of thumb is how you would use the 45. If primarily full swings, use the iron shaft. If finesse shots, the recommendation leans towards a heavier wedge shaft.

You are spot on about ideally getting fit, especially if doing the full wedge swap. Yes, technology has greatly changed, even on the last several variations of Vokey SM models. As for shafts, the one that will matter the most will be your PW and maybe also the GW as those will be more full fairway shots. Others are “feel” clubs so matching what feels good to you is the key. I play light graphite irons (currently the MMT) so I play AMT Red light steel in my wedges as a good compromise to my preferred steel in wedges but not as drastic a transition from my full irons. Good luck. I know you will love the Vokeys.

I use a Vokey 46, 50, 54, 60.
I prefer the Vokey to the PW, though it may be a couple yards shorter. It's easier to spin it.
If you do a lot of bump and runs with PW, its a no brainer to use a Vokey.
I pretty much only chip with my 54/60. I have the F grind on my 46 and 50, but the M and T grind on the 54 and 60 because that's what I chip with. Jordan Spieth exclusively uses his 60 to chip, so his 56 SW is the 10 S grind, which I used to have, but I use the 54 as my main chipping club, so I recently switched to the M with the SM10 because I felt like the S grind didn't give me the versatility I needed.

Since I pretty much only use the 46 and 50 for full shots, I have X100 in those two, but drop down to S400 in the 54 and 60.
So it really depends on you and how you utilize the PW, but probably want to go 6.5 in at least the PW, even Gap. And drop to 6.0 in the SW LW

TLDR: Buy a 45° 10F Vokey with a 6.5 Rifle, Keep your RTXs until they no longer work for you.

The RTX3 vs the newest Vokey, you will have massive improvement based on the grooves of the club giving you hundreds of RPM more spin... Technology hasn't greatly changed, other than the grooves. Vokeys have started raising the center of gravity in the higher lofted clubs because that's what the Tour players wanted. This great technology shift can be achieved by adding some lead tape to the top of the wedge... but is it drastic? If you're on Tour probably. For the average golfer who struggles putting spin on it period, no. But my SM10 60 will flight a few feet lower than an SM6... It's not game changing. Will it fly more predictably in the wind for a tour player from 20yds out? Most definitely... but for us, we're going to get more variability by our swing and our strike, than by a ball flying 2 ft lower in 10mph wind from 40ft out.

It really depends on you. If your RTX3s are doing you fine and they're in great condition. No point in changing them. But if you're chipping and you are like man that's not checking like it used to or like I want it to. Then getting any new wedge, from any brand will help you. If you love your RTX and your getting close to hole, and not feeling like your missing anything. There's no point in dropping $600 just to be "Team Titleist" when your getting along just fine. You'll actually probably chip worse in the beginning cause you'll get check and roll out you're not used to.

But, I can say it is a little satisfying to throw a chip up to the hole and watch it bite and stop instantly and then hear the reaction of your playing partners dumbfounded at how you got so much spin and stop. But it's not the latest and greatest technology and brand new Vokeys that do that. It's technique. Does it make a big difference in the amount, yes. But if you don't have the technique, You could pull out a brand new wedge every chip and you won't get the check.