I think your fuser roller may be dirty or damaged. Try printing several pages that are blank or nearly blank. If it's dirty, the lines should get progressively lighter. If not you may need a new fuser.
You can buy a fuser repair kit for your printer model online. They are usually only slightly more complicated than changing out the toner cartridge. Just be sure to lay down several sheets of printer paper to cover your work area to catch any toner dust.
And a word to the wise for anyone attempting home laser printer repairs. Do not use a vacuum cleaner to suck up any spilled toner. Wipe it up with a damp paper towel or something instead.
Printer toner is so fine that it will sail right through the bag or filters in your vacuum cleaner and then get dispersed forcibly into the air. You know how microplastics are a big deal nowadays? That's what printer toner is. With a vengence.
Toner rated vacuum cleaners do exist but they're expensive, and nobody not doing printer repairs commercially will want to spend the money on one.
after the second or third NOS toner spewing its contents inside a printer, I splurged and purchased a toner vacuum. Cheaper than having the printer tech come out and clean up the mess for me.
as the printer tech explained to me, those NOS toners (at least HP ones) have a plastic piece that wears out when sitting in the box.
What I do is the redneck method, which is take the stupid thing outside, don a respirator, and blast it with the air compressor. Your solution is probably significantly more environmentally friendly, though.
You might be able to see it on the fuser roller but it probably won't be the sort of thing you can wipe off without making it worse.
From the model number shown on your test page (Samsung ML-1660) it looks like a replacement fuser kit is about $30 - $50 dollars depending on the seller. I would encourage you to either replace the fuser or look for a new printer.
According to others it is likely the toner cartridge anyway but where do you find the fuser kits if I decide I need one? I looked on eBay and there are none for sale (in my country anyway).
I didn't think to look on Ali. You even removed the tracking part from the link too 😉 If it turns out to be that, I'll buy one and replace it. Thank you