I wonder if Nancy Walker was meant to be a regular on Rhoda. In the first season she was billed in some episodes as the first co-star, and some episodes as guest-star, and the wedding episode she's billed as special-guest star along with the MTM cast. It seems she left the show a couple of times, but kept being bought back. She was also doing McMillian & Wife, Blansky's Beauties and The Nancy Walker Show during Rhoda's run. My guess is her character kept the show going for as long as it did, but it must have been difficult to include her in the scripts, when Rhoda didn't live with her.
Also, Ida and Martin must have ultimately separated, because Harold Gould pretty much left the show, himself.

Yes Nancy Walker left the show after one episode in season3,she tried two sitcoms that didn't last,then went back to the show,full time seasons 4 and5.
I felt Rhoda wasn't as good as it has been in seasons 4 and 5,due to the customer hire shop I didn't like but thankfully Ida was back.
Harould Gould was in the short lived "Feather and Father Gang" with Stephanie Powers,then he came back too.
Shame so many other good characters like Myrna,and Susie were gone by this time.

And don't forget.In addition to Rhoda,Blansky's Beauties,Family Affair(the last season ),The Nancy Walker Show,and McMillan and Wife,the late Nancy Walker ALSO was Rosie the Waitress in the Bounty("the quicker-picker-upper")paper towels commercials of the 1960's and 1970's.Remember those commercials?