I'm using phosphor bronze but they might have been on the bass for almost a decade.
If you want to sound like a guitar only lower that is the way to go. People who want to sound like an upright bass head for tapewounds. I don't recall seeing flatwounds specifically for acoustic bass so I suggest that might be a niche option.
Scale length matters. The problems with a too short string are obvious. Putting a long scale string on a medium or short scale electric damages the string winding and core. A long time ago I saw a few strings that had the winding unraveled and the string became useless. No idea how true that is today. On an electric if the thread wrapped on the one end is what is on the tuning peg (instead of unwrapped string) I figure things fit.
This might be helpful.
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I have never used JustStrings but other LTGers speak well of them. The lazy thing to do would be ask them which strings, of the type you are interested in, will fit.
Since you are a guitar player, be prepared for sticker shock. I figure garden variety bass strings are $20-$30 a set and have seen specialty strings at $89 for a set.