Another option is to replace the 39t with a "triplizer" ring. If the chainstay clearance permits it without moving the crank out, this is an easy cost effective option. Depending on what you add as a granny, it may or may not require a new front derailleur. (most, but not all left shifters allow for a triple, even if sold as doubles) .
It's something to consider.
Also note that going to a larger cassette will almost guaranty needing a long cage RD, but if you're willing to lve with limitations on the use of the granny, you can often keep the RD with a well configured triple. (it'll depend on the ratios, and what limitations you can accept)
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