I'm not so sure anybody can tell you with certainty if the bath edge is wide enough to sit on safely. If you think it might not be, then it probably isn't.
Which is why I suggested you might want to sit on a toilet seat in one of the smaller Japanese bathrooms.

Personally, I have misgivings against the Toyoko Inn Chain. Certain examples may well be exemplary but as a chain organisation they have historically made ugly decisions affecting customers in same gender relationships as well as some very dubious (and occasionally criminal) decisions and policies regarding building codes - I can't help but suspect that the term "heartful" was a way to avoid certain obligations in place in relation to truly "accessible rooms". On the other hand, companies do change (eventually) and the rebranding of the heartful rooms back to "accessible" is optimistic. I understand that Toyoko Inn do keep bath benches of the type you use at home.

Do bear in mind that other chains - such as the Fresa Hotels, Sunroute too (I think), do tend to have an accessible room. You may need to call them to book it though (or email or use a fax - these are still going strong in Japan). So even if you find a suitable hotel, unless that room is available on your dates it won't necessarily be an option.

Think about what you want to do where you want to be and what you want to spend. I'm certainly willing to help, I'm sure others will be too.