Though they did create the character of Harry Sullivan a few years later specifically because they didn’t think Tom Baker would do his own stunts.
When Baker showed himself more than willing, they wrote Harry out - which I always thought was a shame as he and Sarah Jane played off one another terrificly.
I thought I read that the original plan was to have the Fourth Doctor played by a much older actor, and the character of Harry was added to provide an action figure before Tom Baker was cast. But I could be misremembering.
I guess it depends how closely you are willing to read into stuff w.r.t. Jamie.
The subtext with Liz was certainly more plausible but I’m not sure if that’s just because the Third Doctor seasons were more of a straightforward procedural show, and in that kind of show it’s just part of the language that if a male and female lead aren’t married, there is an implicit will-they-won’t-they. Like in the Avengers, where there was a constant background implication that Patrick Macnee and Diana Rigg might shag at any moment, despite that he could have been her flamboyantly gay uncle.
Harry Sullivan actor Ian Marter went on to be a prolific writer of Doctor Who novelizations before his untimely demise at age 42 from diabetes related heart issues.