I am on Trip Advisor a lot so I did a search on the "all forums" function with the companies you list.
Relaxed Explorer - did not have any luck with getting hits on this company but you may have better luck on the Travel with Disabilities forum mentioned upthread.
Stride Travel - appears to be a travel agent or a company that "links" you with actual tour companies. That often means you're paying a premium as they need their cut too. Actually it sounds like a fancy search engine. I would proceed with great care on this one. If something is too good to be true, it probably is.
Sage Traveling - Sounds like the real deal. They bill themselves as "The European Disabled Travel Experts". They indicate their company founder has a T-4 incomplete spinal cord injury but uses a wheelchair to get around.
Accessible Journeys - Didn't come up with much on this company. Mostly people were asking about it but didn't see any replies from people who had actually used them.
Wave Journey - didn't have any luck on this one
I'll also mention Road Scholar (used to be Elder Hostel). I have done 11 programs with them and they do have some programs with a slower pace. I'll also say that I did one of their programs in Provence this last Fall and there were 3 people who used assistive devices on the tour - 2 used canes and 1 used 2 walking poles. They were able to keep up with the group although they did skip the parts of the tour that had lots of steps. Here is a link to the Road Scholar search engine for "Easy Pace" programs.