I'm looking for a filter that lets through as many wavelengths as possible (from 200 to 5000 nm).
Is that even possible?
What you are looking for is usually called a window, not a filter. You need to check different manufacturers to choose a material that fits your requirements, including not only light transmission but also the pressure differential, chemical inertness, temperature range, etc. Checking ARS' website, calcium fluoride (CaF2) might be what you are looking for.