Flex is a good option if you want to keep using your “written-off” or “old” Chromebook. I have four through my employer. I flashed them all using mrchromebox.tech and two of them work with ChromeOS Flex and the other two work with Manjaro Linux. I’m writing this now from my Manjaro chromebook.

ChromeOS flex runs fine on a real Chromebook. The only thing you might miss is Android apps. But they’re actually really useful on a Chromebook anyway because Android apps are written for smartphones and work well in portrait mode. Since Chromebooks operate primarily in landscape mode, these apps are somewhat unusable.
For example, Firefox and VLC are certainly not useful on a Chromebook. It is better to run the Linux version and you will get a real and better experience.

One Chromebook with ChromeOS Flex runs on my work account and works fine, as well as with Microsoft software running online via Microsoft Office 365. This also works well via Chrome.

I received an update to ChromeOS Flex this morning. I am very comfortable with it.

As for the other two Chromebooks running Manjaro Linux. I chose this because of the power management and touchscreen. This also works with Ubuntu, Xubuntu, etc. or Linux Mint, but the audio causes problems with VLC and Celluloid. I don’t have any problems with that with Manjaro. I’ve also tried Endeavor OS, which also works well, but it’s a bit cumbersome for me to update.

Chromebooks have great battery life, and 12+ hours is no exception. They can easily get through a day.