Aaaaand who's going to crawl under the sink/up the basement ceiling to unclog the kitchen's sink?
So glad to be home. /s
@hypolite Yep, I tried hot water, then did it all wrong and failed my chemistry test (lesson learned).
Then I went for the mechanical method, unscrewed the pipes, used a home-made drain snake and failed again.
I could completely remove the pipe that goes through the kitchen floor hoping to access the basement century-old cast iron pipe where the blockage seems to be but I feel like I'm just going to waste my time and end up having to call the plumber anyway.
So plumber it is.