Despite your concerns about available ratios on a wider freewheel, that's the best first step. If your system is non-indexed you can go to a 13-28 7s freewheel which is the same overall width as a 6s. Odds are that with a longer chain you can skate by on the existing RD, by giving up the use of the inner chainring, with the outer few sprockets. With your 42/52 chainset, you'd need 25t of take up capacity, and I doubt you'll lose anything except the 42/13 crossed combination (which you wouldn't use anyway).
The reason I suggest starting with a freewheel swap, is that that's something you're going to need anyway, and if it works out you're that far ahead of the game. If not, you can always pull the plug and go for a crankset. Use a gear chart to compare all the possible options before spending any fough.
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