I feel you, slow drains are so frustrating. I had almost the exact same issue a few months back. Did the DIY stuff too, but it only helped for like a day. In the end I decided to bring in a plumber because I didn't want to make it worse or deal with a bigger mess later. Found a reliable local team through https://cbsrenovation.nl/consumer/plumbing-services/ — they came quickly, snaked the line properly, and charged fairly without pushing extra work. If it's been ongoing, might be worth getting someone to check it sooner rather than later. Good luck!