Instead of using a cronjob you can put the device into sleep and use the RTC to schedule the next wakeup (see [1]). This takes only very little power, as the device is only turned on for mere seconds and sleeps the remaining time.
Some Kindles can easily be jailbroken [1]. I have two jailbroken Kindle 4 devices and they're still great. Both for reading (though you have to sideload books) and as e-ink dashboards [2]. A Kindle 4 can run for ~ 28 days on a single charge, refreshing the screen every hour.
I've personally only tested it with a Kindle 4 NT. I haven't had any reports of people using this on other Kindle devices, but in theory it should work as long as the device is jailbroken.
Highscalability.com was great! I still follow their RSS feed but there's not a lot of new content nowadays. If someone knows a similar resource I'd love to hear about it!
Flutter for web is quite rapidly improving in all the areas you mentioned. For right now I think it's already very suitable for highly complex browser applications such as a Figma, Google sheets etc. Load times aren't the best due to a quite hefty WASM download, but for _applications_ that you typically open only a couple of times per day and where you work in for several hours, that initial loading delay really doesn't matter.
Just don't expect to use it for an E-Commerce site or something like that.