I wrote a long response replying to everyone, but it's still pending approval for some reason
It's a 7 speed cassette, not freewheel. Goal was basically to get lower gearing so I can take the bike on short tours. I initially just wanted to get a bigger cassette, but ended up also getting a 50/34 crankset and a more reasonable 13/28 cassette.
If you Google "SJSCycles Alloy Compact Double Square Taper Chainset - 50/34T", it's the first result. I think everything should fit (7speed compatible, JIS square taper), with the only risk being that my BB is slightly longer (it's either 113mm or 115mm, whereas the crankset was designed for 110mm). According to comments on the product page, that should be ok.
I also toyed with the triple crankset idea but it seemed much harder to get right, with a high chance of having to change the BB and/or front derailleur. I don't plan on doing long tours or using heavy loads, so I think this should be a good starting point. I think the next step if this isn't sufficient is to go with an old MTB rear derailleur and a larger cassette.