I have a mid-2009 (2.13 GHz Core 2 Duo) with Snow Leopard which I use infrequently for vintage, specialized software and some old, old games. I upgraded it rather inexpensively as a small hobby during the pandemic with a (at the time cheap) 512GB SATA SSD, aftermarket replacement battery, and 8GB of RAM so it boots rather quickly.
On battery, I get approximately 4 hours of moderate use, sometimes less depending on what I do. The SSD really made a difference on battery life. The only "tweak" I needed to do was turn off the SMS sudden motion sensor, otherwise, it's been running like a top for the last few years. I periodically will play some OLD PowerPC software and games on it (thanks Rosetta!). Heat isn't a big deal, the bottom case was replaced YEARS ago under a repair extension program and hasn't had an issue. I opened it once in the last few years for a light "dusting" and it's been running just fine. I do run it with FileVault which runs rathe seamlessly on the SSD.
Anecdotally, you can, technically, run it with ChromeOS Flex (I've done it via USB drive) but it's not what I think anyone would refer to as a terrific experience, especially if you're not running it with an SSD. If you have a specific need for a ChromeOS device, then it works great for most functions but not something I would plan on using beyond a few days. The battery life really takes a dive, not nearly as refined as just running old macOS.