Leave the big ring, Absolutely no reason to mess with that. You should have a 39T middle ring, good. Go with a 26T granny ring. Try that on a hill before you change out the cassette. Ignore the specific recs for chain wrap, etc. I've run a 53-39-26 front and a 11-40 cassette on our tandem for many years, no problem. Only an idiot would use small-small. We should presume you are not an idiot. I had to use the MTB RD and a Wolf Tooth to use that 40T cassette, but it all works fine. On my single my low is 30-26 on a 9-speed triple rig. That's been good for long pass climbs for a few years. I'm 78, no great athlete, just ride a lot. If you need a Wolf Tooth to run a bigger cassette, it just lowers your RD pulley, so you have to mess with chain length, but no big deal. Works fine. But I'll bet you'll like the 26T granny. It's cheap fix.
On my tandem and single, I only use the granny on the 4 largest cogs and don't use the big ring on the three largest cogs. The middle ring can run the whole cassette. On my single, I can go big-big though that's a pilot error. On the tandem I can't go big-big, but I have a Shimano Flight Deck - I can't see the cassette at all. You wouldn't need one on a single.
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