Sorry for the long question, feel free to just go off of the pictures!
Background info.: I've had him for about 10 months. He gets bright indirect light in a SW window, watered with rain or de-mineralized water when the soil feels dry and the pot is lighter. He's pushing out lots of big beautiful new leaves, I repotted him in late June and his roots (healthy-looking) are already starting to come out the bottom of the new pot. The soil is coco coir with a generous helping of perlite and clay.
Miscellaneous info.: This is a very dry house (north of the Netherlands), he sits in front of a window so it can be drafty, IIRC this started during or right after a heat wave (~35°C/95°F) but I made sure he got no harsh sun.
Problem: Starting in late July, he's been getting these orange spots. They are very specifically only on the tips of the leaves, and so far only affecting his middle-growth (not the old or new leaves). The spots start off yellow, then become bright orange, with a translucent outline that is visible when holding the leaf up to the light. There are also a few "stippled" clusters of tiny translucent spots (no orange) on some leaves.
It's been almost 2 months, the spots haven't noticeably gotten bigger, but there are more affected leaves now. A couple of the old spots dried out and became papery and sheer white (maybe from attempts to treat). Meanwhile, the tips that were yellowing have dried and browned (still with a yellow outline), so maybe there are two problems going on at once. The white specks in the photos are dust, I haven't seen any pests. The brown margins on the bottom leaves are old fluoride burns.
I figured it could be fertilizer burn, as I had been giving him diluted 3-1-5 fertilizer pretty much at every watering. I flushed his soil thoroughly the day I first noticed the spots. I also hosed the leaves down, which wasn't the smartest thing as I later found out, but I did dry them right after. I haven't fertilized since July.
I stopped misting.
More spots appeared, so leaning more towards fungus, I sprayed the tips of some of the affected leaves with a mix of water, Dr. Bronner's and baking soda (twice). Didn't seem to do much. I made sure to catch any falling droplets with a towel and not let it run down the leaves. I've been careful not to spread anything by touch as well.
I read that underwatering can cause yellow tips. He'd been getting dry much faster than before, in about 3 days as opposed to 7/10 days, but I didn't want to overwater so I'd let him dry a couple days more. I increased my watering since then.
Leaning towards fungus again, I figured that drying out the tips aggressively might work. I spot-applied some 3% peroxide, barely diluted, on some of the orange spots. I'm not sure if this had the intended effect, because the burnt-ness of the treated spots looks really similar to the spots themselves.
Figured I should finally ask for an outside opinion! It hasn't spread to any of my other plants. I think the fact that it's only affecting the tips of the middle-growth has to have significance. Also, none of the spots have had a brown center yet (so maybe not fungus?). What do you reckon? Link to extra photos (including whole plant)