Congrats on the new house, but yeah, that smell is a classic sign of a failing line, probably roots or a collapse. Don't waste your money on those "miracle" drain cleaners; they usually do more harm than good on old pipes. I had a similar disaster and went with https://superdrainkingston.ca/ to get it sorted. They used a camera to show me exactly where the break was, which was super cool but also gross. They were really efficient and managed to replace the line much faster than I expected. They were very honest about the pricing too, no hidden fees or "oh, we found another problem" nonsense. Just get it inspected properly before it backs up into your kitchen!