Hey, I've built a similar device, also with a washing chamber. Somewhere you said you're using a water bath, but shouldn't you use KOH or NaOH solution instead for washing out acid fumes? I'm not a chemical expert, but those bases neutralize acids. I don't know if plain water will help a lot.
You mentioned the pressure in your reaction chamber getting too high, I have a 5L PEHD fuel tank as reaction chamber (plan is to mix 2L S + 1L F or something like that), with quite a big PVC tube that goes to the self-built washing bottle (~1cm inner diameter), of course everything's airtight. I think it's big enough for the pressure, but I have done no tests yet. How thick were your tubes in that test?
The plan is to pump the formic into the reaction chamber using a peristaltic pump with acid-resistant tubes running @ ~ 9V (with 2x 18650), time controlled by a timer relay. I've already tested that (with water) and it works as expected.