The big watershed question in matters like this is, do you take pictures while diving, or do you dive to take pictures? A G7X with a single strobe can go either way - you can clip it off and just enjoy the dive, pulling it out when you want to take an occasional snapshot for memories, but it can also serve as a centerpiece of a dedicated photography dive, where most of your time is occupied by looking for photography subjects and opportunities. An A7R IV with a pair of strobes, however, falls squarely in the latter camp - compact as it is, it's still big, heavy, draggy, expensive, and in general not something you just bring along unless photography is the main aim of your dive.
If you do dive for photos, then housing your A7R IV - which will cost somewhere around $10k, factoring in housing, ports, lenses, wet lenses, strobes and other ancillaries - will give you considerably greater detail in your photos, much faster and more reliable autofocus, and, with a dedicated macro setup (i.e. Sony 90mm), much better macro capability. On the other hand, your rig will be several times heavier (think 10kg or thereabouts, although you can make it neutral in the water with floats), draggier, and all-around more difficult to maneuver, especially in tight spaces. You will also get considerably shallower depth of field in your shots, which is limiting in macro, and your flash sync speed will be limited to 1/160s (1/250s with a TTL-capable trigger), which can affect your ability to shoot sunballs.
As far as flexibility goes, you can dive it with 28-60mm and wet lenses, just like your G7X, although the WWL-1 wet lens required to do so is still quite bulky and heavy. Alternatively, you can shoot it with dedicated fisheye (Canon 8-15mm/Sigma 15mm), rectilinear wide (Sony 12-24mm, 16-35mm, Tamron 17-28mm) or macro (Sony 90mm, Sigma 105mm) lenses. The body and housing themselves are not particularly big, but ports, and especially long strobe arms and floats definitely add up to overall bulk.