Thanks for your contributions, lavandula. Maybe I should have done breakfast there!
Mainz is a city of some size and sprawls a bit. Also, the main station lies somewhat outside the core of the city. Still, most of the hotels and rooms are there at the station, and that's where most people choose to stay, I think, so walking to and from most tourist destinations in Mainz over a few days can definitely put some miles on your tennies. And the Römerwall would only add to the walking.
"If I had to do it again: I liked it (The Römerwall) enough there, but if I expected to go into the city at all, I wouldn't stay there."
I get that, especially if you're there for 5 days with a group of people and plan to get around on foot. I've always considered the old-town core locations - which don't offer all that many rooms in total - preferable to the others for most visitors... the Ibis and the Hof Ehrenfels were good choices there IME. The Hilton, further out down by the river is a possibility there too. If you do visit Mainz again, this location would minimize the necessary walking. AND, there's a second train station in Mainz - the "Roman Theater" station, which is how you would approach this location in the first place; get off there, and you won't be schlepping bags far at all.
Personally, I've found most locations very doable in combination with public transport when needed. Over the years I've stayed in several different places in Mainz...
Two different places right by the Mainz Hbf station, where the main concentration of hotels - including several chain hotels - can be found (and where most visitor choose to stay, it seems.) I don't find the area cozy myself. But there are lots of restaurants in the area, and if you do stay here, the local buses will take you to the old town in 6-7 minutes. Alternatively, board the train to the Roman Theater station (maybe 4 minutes) and you'll save a lot of steps.
The two different places in the old town zone (near Augustinerstrasse) mentioned above.
The DJH hostel (way out by the Volkspark) which is probably to be avoided except for group travel.
The Römerwall (on this most recent trip) which involves a walk to the Hbf station (around 8 minutes IME) and the bus or train (4-8 minutes) from there into the old town. At 73, I did not find the uphill walk from town/station to hotel intimidating, but I "cheat" - I pack light in the extreme, and I did not do any self-catering or haul any grocery supplies around during my 2-day stay.
The Alte Post Garni in Mainz-Gustavsburg (a suburb just across the Rhine, also on this most recent trip.) I rode the S-Bahn straight to the Roman Theater station in Mainz (4 minutes) to reach the old-town zone.
The Moguntia (also on this most recent trip) is about 10 minutes on foot from the Hbf station, a couple blocks further out from the station than the Holiday Inn and the Best Western. Same public transport options from the Hbf station as above.
But let's say you are staying at Mainz Hbf, the Römerwall, or the Moguntia, but you don't want to bother with buses/trains etc... Mainz has a large network of mostly-quiet pedestrianized streets that stretches from the old town center almost all the way to the Hbf area - stick to these streets (in pink on the map below) where things are quieter and city traffic stress fades to conserve your walking energy. From the station's entrance, walk straight out a couple of blocks to Neubrunnenstrasse and enter the pink zone... click to expand the map; note that the two in-town train stations are shown as well (DB):
https://www.orangesmile.com/common/img_city_maps/mainz-map-2.jpg