As the question says, I'm looking for a word similar to, but not quite the same in meaning as, "polymath".
A polymath tends to carry a sense of either being a generalist (doesn't imply any sense of specificity), or genus (not relevant), or ideal/rennaisance person (not useful).
Portrait of a person I'm trying to describe:
Did a qualification as an engineer.
Decided they wanted to write a software for something they needed personally. As best I can tell, digested professional quality and competence at a suitable language for the task, from a standing start, in 4 months (I do mean professional standard - including documentation, naming conventions, comments, explanation being "I might forget what I did"). No interest in other programming or software, as best I can tell this was motivated by needing something not found on the market after wanting such a thing for some years. Pure personal "Make it so".
5 years later got cheated by their employer in a complex area. Took employer to court. Dismissed specialist lawyer saying he wanted to not have anyone else's less-than-needed quality to blame if there was a poor outcome, researched IP law, legal procedure, and case presentation for 3 months (never had any legal background), presented a case that judge commended as one of the best he had heard from a layperson. (Precise and lethal!)
Took up photography as hobby. Placed second in his first Photoshop contest (a moderately well known one) about 6 weeks after deciding to enter, having never used Photoshop or any other significant digital imaging software before. Doesn't seem to have used it other than in a couple of competitions and to help me with family photos a couple of times.
And so on.
This is someone who figures if he needs to know something, he will have absorbed it in a few weeks and be better able to do it than most professionals in 4-6 months (although likely to lack knowhow outside the specific goal that motivated the research). Picks up new fields as life needs it. Doesn't see it as wasted time if not needed for more than the one present task (my guess is, wouldn't have gone that route if he didn't feel the use justified the time).
To be clear, he doesn't do this sort of thing often at all. Maybe a couple of times a decade from what I know. But its a skill he can apply to many subjects if a need arose.
If he was told by a doctor of some odd condition, he would most likely know more about it (including key findings from most of the relevant latest research) a week later, including knowing how best his care should be managed in dealing with it, and would probably be right, including background terminology and general medical views. Would use it to help him discuss it (almost as equals) with his medical consultant, despite never having looked as medical research before and ensure he was happy with a major decision affecting his life and/or family. That sort of person.
So I'm looking for a word that implies/describes this sort of ability or characteristic - someone has the skill of _ learning new skills across a wide range_, rather than general mastery, genius, or whatever. Ideally also implies having high levels of competence at them if desired. Polymath seems wrong. Also too abstruse a term for a very down to earth person.