Saw a post on Hackaday a while back about a guy running a CNC router off XP for like a decade, same deal, dead CMOS battery had his dates all wrong and nobody noticed till file timestamps went nuts. The Pi route is fine but you'll need a USB to serial adapter anyway, so the real COM port thing doesn't fully go away. For a label printer a Pi is honestly overkill, a $30 thin client with a real port would do the job.

A 9 hour battery on a 17 year old laptop? I'd want to see that with my own eyes before I believed it. Replacement batteries are a gamble, sometimes they claim way more than they actually deliver. Plus those old ThinkPads had like a 9 cell battery that sticks out the back, it's basically a boat anchor. The real question is what were you doing on that 2022 laptop, cause some of those new ones burn through power just sitting there with the screen on. I'll believe the old laptop is decent, but calling it a full on victory over new tech feels like a stretch.