Why does my portable AC suddenly smell like an ashtray?

That white stuff on the coil is probably not frost if the unit had been off for hours. Real frost only shows up while it's running, and it melts fast. What you're seeing is more likely dried condensation residue, basically dust and mineral gunk left behind after the water evaporated off the tubes. It's really common on a unit that's been through one season. Not a big deal on its own, but it does tell you the coil has been collecting stuff, and that matters for your smell problem.

Here's what's going on with the cigarette odor. The coil inside a portable AC gets soaking wet every time it runs because it's the cold side where humidity condenses out of the air. Anything in that air, including smoke that got pulled in through the window kit or even a fire pit night from the neighbors, sticks to that wet surface. Then the water dries and the smell bakes into the dust layer on the coil and the foam around it. That's why it lingers for weeks and why it's strongest at the machine. The unit basically stored the smell. Rinsing the filter alone won't touch it, the filter is usually fine in a case like this. You need to clean the coil itself. The easy way is a no-rinse evaporator coil spray from any hardware store, you just spray it on the coils and it foams up and drains away as the unit runs. If you'd rather not buy anything, a spray bottle with mostly water and a splash of vinegar works too. Spray it on, let it sit a few minutes, then run the unit in fan-only mode for a good hour to dry everything out. Since you already had the panel off, that's the same amount of disassembly you've done before.

One more thing while you're in there. If there's a drain plug on the bottom, pull it and let any stale water out, that pan can hold some gross leftovers. And after every run this summer, give it ten or fifteen minutes in fan-only before you shut it off. Keeping the coil dry between runs is what stops the smell from coming back.

I had something similar with a used unit I bought off a buddy who smoked, and the smell hung around for ages even after I cleaned everything I coud reach. Turns out the plastic housing itself soaks odors up too, not just the filter and foam. What finally helped was leaving it outside in the shade running fan only for a whole afternoon. So following this because I'm curious if the spray alone does it or if the smell is baked into the plastic shell as well. Also is it worth doing the vinegar thing more than once if the first round only dulls the smell? I'm not very handy either so hoping there's a low effort answer.

I'd push back on the vinegar spritz suggestion. I tried the water and vinegar thing on my old unit and it did basicly nothing, the smell was back in a day. The baked-on gunk on the coil needs something that actualy foams and lifts it, the no-rinse coil cleaner is what finally worked for me. Also not sold on the fan-only afterwards fixing anything long term, mine still got musty every few weeks til I stopped running it with the window kit leaking and sealed that up properly. Honestly some of that smoke smell may just be coming in the gap around the exhaust hose every time it runs, not stored in the machine at all.