Here the fully re-recorded version from 8 years later is in the same release group: Release group “Actual Fantasy” by Ayreon - MusicBrainz
Here it is a new release group: Release group “Fearless (Taylor’s Version)” by Taylor Swift - MusicBrainz
What’s the proper way?
New Release Group as it is a new release of new recordings.
I was about to post a Taylor Swift example and then noticed you had already done that 
(That one was actually discussed in the forum somewhere)
The Actual Fantasy release should be split into two separate Release Groups. They can be linked with a Release Group relationship of “Re-recording of”
I wouldn’t object to a separate release group, but the reason that I hesitated about separating Actual Fantasy into two RGs myself in the past is that it’s not a complete re-recording. The main change is to replace synth drums with real drums. That’s not the only change, because there are some new guitar and bass parts too, but e.g. I think the original vocal takes are still used: Actual Fantasy - Wikipedia
So it’s all new recordings on MB, but that also applies to re-issues with new “polished” mixes, which don’t get their own RG. Felt like an edge case to me: but maybe in that case it’s still better to separate than to merge.
(See also this RG: Release group “Night” by Gazpacho - MusicBrainz
The 2012 version has some parts re-recorded – drums again, for example – but not everything, and the intention from the artist seems to be that it should be treated more like an improved mix: Astounded by Sound!: Gazpacho - Night
Unlike the Ayreon example, the title was not modified in this case.)
There is always a line that is grey. The Actual Fantasy ones seem majority new. And such a new effort they used a new title. Would be a clear separate RG for me.
I also think you are right that the Gazpacho example seems more like improving a mix. The majority of the music is still the same. More “improved” than “replaced”.
A new RG is the way to go. Putting them all in the same RG as the original album would be awkward because there are multiple releases of the Revisited version, and Revisited has its own reviews and other RG-level URL relationships that are distinct from the original’s.
Also, based on your description I would say it’s only a new mix. These are multi-track recording, and they replaced a small number of instrument-specific tracks (out of necessity), then did a new mixdown. Some remixes do more than this, others do less. Remix might be more accurate than re-recording.
I think the remix type on MB is intended for remix albums in a more precise sense, i.e. radical reworkings of an artist’s material, often by other artists. (An example.) An improved mix of an album, intended to sound “the same but better” instead of obviously very different, is not typically given its own release group. (There is a small section about this in style, admittedly on a semi-official page. It is unfortunate that there aren’t two different words for these two very different kinds of remix.)
On the other hand, full re-recordings do usually get their own release group, so it’s a question of where to draw the line. In this case I would vote for separating (in part because as you say a lot of external links apply to the Revisited version specifically, while still not being specific to an individual release). If it really was just a remix with no re-recording, I would vote for keeping a single RG. (You might still be able to make an artist intent case based on the title… although without the re-recording I doubt the title would have been changed, and I know at least one example where a polished mix with a new title still keeps the same RG.)
Since one of the releases of …Revisited was missing this part of the title, I used the excuse to create the new RG and propose to move this release into it: Edit #149021788
Since this is a live discussion, I’ll see what happens with that before attempting to move any of the other releases (or removing the “aka” disambig and alias from the existing RG) – but feel free to go ahead if you are bolder than me! I used “re-recording” for the RG relationship for now.