Thanks for your reply, R0J0hound!
I had already found some of your helpful posts and .capx files here and there but wasn't sure they'd suit my project. Turns out they do!
However for now I've been testing another of your solutions for colliding/sliding, the one named event_motion2.capx (https://www.scirra.com/forum/event-based-movements-amp-collisions_p719471?#p719471). I've had a bit of a hard time getting it to work in my project exactly as in yours, but I found out why: my player's sprite was 16x16 pixels scaled up to 128x128 and somehow it made collisions tighter or less tolerant... Anyway – I've changed my sprite for an actual 128x128 image displayed 1:1 (thus the collision polygon is 8 times bigger, and maybe 8 times more accurate?) and that did it! Now it's working as intended. The player collides with the walls but can slide sidewards a bit if it hits the extremity of a corner, and you can slide flawlessly along a wall that's already blocking you.
I've looked at your latest .capx (the one you pointed to above) but I'm not sure I should use that one. Does it always handle the player's sprite as a circle when it collides? More importantly, can the size/position of the circle (relatively to the sprite's image) be tweaked? Oh and lastly, how do you think both of your methods compare in terms of processing power?