My Pixma mg5650 isn't powering on, what else can I try?

I've had my pixma mg5650 for about 8 years now. About a year or so ago, I started getting the dreaded B203 error ( - IIRC; it was definitely B20X). I seemed to be able to fix it temporarily by unplugging, waiting (sometimes a day or so), and plugging it back in (I didn't have much time or strength due to health complications to get any further into fixing it). I also didn't need the printer much at the time, mostly I was just printing a test page to keep the ink flowing so that the print head didn't clog.

Eventually, though, it just stopped working entirely. No lights, no sign of life, nothing - as if it weren't plugged in.

In the meantime, I've gotten around to taking a closer look at what's going on. Here's what I've done:

• The "standard tech support suggestions": checking to make sure it's actually plugged in, trying a different power cable, outlet, etc.
• Connected it directly to my computer via USB (I used it exclusively via WiFi otherwise). Nothing, the computer doesn't react to anything. Nothing appears in the device manager (or, to be exact, usbview, which I find a lot more practical for things like this).
• Checked the power supply. I tested the voltages, it seems to output what it should (at least according to this video, it seems to be hard to come across information for this specific printer, this video popped up when I entered the power supply's model number), 10.5V and 8V, because it's in standby mode (the video explains that it can go up to over 20V). I disassembled it (obviously following safety precautions) to see if I could find anything that seemed off, but everything seems OK.
• checked the board with the controls and the display. No obvious faults there either.
• Disassembled it down to the logic board. The only thing that seemed a bit off was that there was some weird resin-like material on the board:

The resin is the orange-ish stuff at the top left and bottom right. On the top right are the 5 power supply pins, the two large soldering "joints" to the left is the largest capacitor, to the left of that connectors for I'm 95% sure motors. On the bottom right, where the other orange-ish stuff is, those are the USB connectors. There is no voltage there.
Mind you, it seems to me like this must have been on there since day one, because it's completely dry and not sticky or anything, and there's no trace of anything similar anywhere on other nearby components. It's fairly easy to scratch off, and underneath what I've scratched off, everything seems fine, at least to my untrained eye. It definitely doesn't look like I would expect it to if this was something that came from excessive heat.

• Checked the most important cables. The board is definitely getting power, two of the capacitors for the two power supply lines are charged when it's on (also with 10.5V and 8V respectively). So power supply shouldn't be an issue, unless somehow the power supply can't get out of standby. The cable connecting the logic board to the board with the controls also seems fine, no issues with contact or anything.
• I also checked if there's any voltage going to the board with the controls and the display, and there's definitely something there. So the board is getting power and something is going on, just not what should be.

Any clue what else I could do? Or am I SOL? I mean, worst case scenario, I have to buy a new printer, and this time look around a little more carefully in hopes of getting one that lasts quite a while (and if anyone has any suggestions, I'm open, I have heard Brothers are good, but haven't done too much research yet). But maybe I missed something obvious and I can still save this printer? To me it seems like it could be a number of things.

The printer is almost identical to the one being disassembled in this video - there seem to be a lot of pixma models with only minute differences.

I'm decently well experienced with repairing and upgrading phones and especially computers, but I've never tried to repair a printer, so I may be missing something obvious. I guess I have to start somewhere :)

EDIT: the logic board is this one here.

EDIT: as requested, pictures of the power board (I can't seem to add it in comments):