I used my Vega 64 since AMD launched it up until November 2018. I got the RTX 2070 because I helped my kid brother build his first pc & gave him the Vega.
I would not bargain on g sync working perfectly on every freesync & this needs to be considered. Thus far Nvidia have only 'certified' 12 monitors - you can try g sync on every freesync but according to nvidia there are 12 out there which they believe are the best.
In my opinion its not worth going out there & buying a g sync monitor + an nvidia card. If youve got the money by all means but it is a very expensive investment. I would only do this if I had the money & was able to sell the Freesync for a good price.
Like ive said, ive owned both freesync & g sync - they are very similar at the high end.
Whether you go for Vega 64 or 56 is up to you, but from experience I can tell you that a 56 is the better option between the two. Even just a quick overclock will bring the 56 to a stock 64 performance.
Watch to see what a heavily tweaked Vega 56 can do
https://youtu.be/w6gpxe0QoUs
Watch to see vega 64 vs rtx 2070
https://youtu.be/gLgBHo-N1rA
One thing to note that is important, with vega cards you dont need to enable performance mode when gaming, the cards will consume significantly more power but dont offer much performance, I usually used power saving or balanced modes - performance changes were very minimal but power savings were drastic.
The vega 56 is the better value vega to go for, if you have the money get a 64 but the performance difference is marginal.
(Your power usage / cost figures are inaccurate, I can tell you from experience that I never noticed my electricity bill being more expensive, definitely not to the point of £50
)