Hi Adam,
Yes, the photography board has unfortunately dwindled, although we have a relative new comer to Sony @klmall (Kathy) to thank for keeping this forum active.
I've taken my A7r2 twice to WDW and haven't really had an issue with shooting dark rides and your A7r3 has better focus. I personally appreciate the higher resolution and DR.
The bigger question is what lenses? I used the 24mm 1.4 GM, the SZ 55mm 1.8, the 85mm 1.8 and also brought along the 105mm 1.4 Sigma, Laowa 15mm 2.0, and my Rokinon 8mm fisheye just because it's so much fun.
Hope all is well - are you still writing a blog?
DSC06510_LuminarAI-edit.jpg by Adam Brown, on Flickr@havoc315
Also just bought the 20mm 1.8 - haven't even taken it out of the box yet.
Food for thought - if you go with the A7R3, the 135mm + cropping ability should be just fine for AK.
If A9 - then bring the Sigma zoom.
DSC09033.jpg by Adam Brown, on Flickr
DSC09023.jpg by Adam Brown, on Flickr
DSC08957.jpg by Adam Brown, on Flickr
DSC08953.jpg by Adam Brown, on Flickr
DSC08802.jpg by Adam Brown, on Flickr
DSC08686.jpg by Adam Brown, on Flickr
DSC00074.jpg by Adam Brown, on Flickr
DSC00684.jpg by Adam Brown, on FlickrDSC00684.jpg by Adam Brown, on Flickr
That looks a lot like a complex I know in Tahoe? Maybe not enough snow though.