Valchoose 4PACK Stainless Steel Woven Wire Mesh Never Rust, Air Vent Mesh 11.8"X8.2"(300X 210mm), Hard and Heat Resisting Screen, 1mm Hole 20 Mesh Easy to Cut for Rodents Filters Window DIY Projects

Then yea, that's probably it. Add a woven wire mesh like this that has small holes so gnats can't get in or out the ventilation areas. Then tape some sticky traps for planters in places that isopods can't get to. If you do place them on the walls of the terrarium, I find that bending the "sticky head" part downwards will catch the flies better. You can maybe even dangle it from the top like a pinata. This will take a few weeks to maybe a month or two to fully eliminate them. Or you can use these Mosquito Dunks that supposedly works on them and does not harm other creatures in the terrarium. I haven't tried it myself but I've been told it works. If you already know all this, apologies for the long ass paragraph.