@Mentha, thanks for offering! I've collected too many perforated Monsteras; found two more big ones locally. (I don't know if they're obliqua, friedrichsthalii, or adansonii. I read they're supposed to be synonymous but the leaves are different; one has much pointier leaves than the others, with smaller slits). But if you need to send them away, sell them on eBay? I'd be happy to adopt another, too! That little one is doing OK right now. (Making a new leaf, looks clean.) I've learned a lesson: DON'T go crazy stripping "root bound" plants, and pot up according to root size, not plant size. (I don't know how plants survive suggestions like Figure 2. here; a nursery owner told me they actually don't touch the outer roots at all when repotting, and just add more soil around them. Their plants look great.) I thought I was "helping" them by trying to put them in coarser soils like suggested here, but they didn't enjoy the excessive root pruning when I tried to remove the old soil. And I used way too much hydrogen peroxide in trying to compensate for their new, over-large pots. (Something like 1-to-4 rather than 1-to-11 of 3%; surely burned them.) Anyway, I've repotted again (downsized) and am letting them rest and recover. P.S. nice Skyrim icon :D

Mine are all outside but when they get a bit too dry and stressed some of the older leaves will develop that sort of blotchiness. I think the plant decides it has too many leaves to support and begins the process of shedding some of the older ones. I never fertilise mine, as semi-epiphytes they don't need much. Of course a bit wouldn't do any harm, especially if they're in pots with restricted root space. In a garden falling leaves and insects are enough.

I'm not experienced with that. I've seen several similar-looking Philodendron 'swiss cheese' but never had any. Last summer I planted 2 ailing indoor plants outdoors in ideal spots and they perked up and ended up doing well. Both were aroids: EE's. This spring I did that with 4 more indoor plants that were on the fence: 1 Caladium, 1 Elephant Ear(Yautia?), 1 banana, & 1 Ligustrum bonsai and they've all perked up except for the Ligustrum. It is doing worse. I removed 50% of the Ligustrum roots in the fall so I could place it in a smaller pot. Maybe that was too much of a shock. All but the Ligustrum have cages or small fence around them to keep rodents from chewing them. (BTW - this evening I found a nest of rabbits just 12 inches from the Ligustrum) The plants all have ideal light, wind protection, and water. 5 out of 6 is not bad and I am not certain why that has worked. If it's the warm season in your part of the world and you have an outdoor growing area, that might be worth a try. Protected Yautia or Eddoes...not sure: Ligustrum on top right:

Yes green balls are fertilizer. you said no repots, so Im guessing plant is still in its original store soil. Those are usually way too water retentive. A lot of people on here either use Al’s 5-1-1 mix for tropical or soil mixed 50% w/ 50% sifted perlite or pumice. 5-1-1 has 5 parts pine or fir bark, 1 pt. Sifted, coarse-grade perlite & 1 part peat. The issue w/ your Monstera looks like bacterial leaf spot from too wet soil, but I could be mistaken.. others on here may be better at diagnosing. These grow under trees & climb their trunks like a vine in the wild, south of the equator. Your water & sun accommodations sound good, as long as you keep it where it’s at, that light would be similar to what it wants, which is part shade.