Is it possible to build a witch farm without a witch hut?

I've done some tests, experiments and research and it seems like there's nothing that isn't already in the comments or generally known, so I'll summarise it here:

Witches spawn in witch huts. Those are not an option for your Superflat Survival.

Witches spawn in raids. The bug MC-139817 ruins that for you, because you can't get patrols in Superflat worlds, so no "Bad omen", so no raids.

So the only option left is regular spawning. That spawning cannot be caused in any way that doesn't also allow zombies, skeletons and creepers to spawn. You could use their AI to separate them, but you'll want to kill them all anyway, so you might as well handle them all the same.

Here are a few small optimisations you can do, apart from the usual mob farm optimisations like building low down, no blocks higher than the farm, killing fast, lighting up around, etc.:

• build the farm outside of slime chunks, because slimes are part of the mob cap
• do not use grass with light on it (like sunshine), because if animals spawn, that covers possible spawn spaces.
• make the spawn room 2 blocks high to prevent endermen from spawning
• place slabs to prevent spiders from spawning, in this pattern:

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Of course that only works if your method of transporting the mobs away is not hindered by that. If that helps, you can dispense water into slabs since 1.13.

• You need some killing mechanism that kills witches efficiently. A drop is not very efficient, because they fall for a while and if you build low, they also need to be transported up for a while. A crusher is efficient, as well as a minecart crammer, also magma blocks and/or wither roses.
• You can also kill them manually using a Looting sword, that gives you more mob drops.

This is technically a different question, but I'll shortly answer it anyway: The wiki (archive) lists all ways to get redstone. Apart from generation and witches, cleric villagers can also trade you redstone. I would recommend checking what you can trade for emeralds (a huge sugarcane farm is usually pretty effective or bamboo or trees for sticks) and using that as a main source of redstone.