That's the one thing that surprises me the most in your post. Any idea if it was actual browser engine issues, or running on inadequate hardware?
I believe the device has 512MB RAM and a quad core 1.1GHz A7 - so not exactly rocking the hardware. But the browser engine didn't have things like some basic Javascript support, near as I could tell - Discourse forums wouldn't load properly, grumbling about how I ought to enable Javascript. And on that little CPU and RAM, anything "faintly modern" just wouldn't work right at all.
Not that you can see much, it's a 240x320px screen.
There are some higher end KaiOS devices out there I debated, but spending a couple hundred on that level of functionality just didn't seem a good use of funds to me.
The problems I see with KaiOS are 1) lack of aggressive development to fix bugs/features, 2) lack of vendors upgrading the OS on new handsets, 3) lack of vendors upgrading the hardware to run newer versions of the OS. And the market doesn't really care (or have a choice), they just need the cheapest phone possible that can check the weather forecast or email.
Until both developers and vendors give a crap about building efficient technology, they will continue to churn out bloated, buggy software on cheap hardware. Even Palm Pilot was more advanced than the crap churned out today.
I would pay a lot of money for a Palm Pilot designed for 2022. The team that created Palms were gods.
iirc opera mini browser did something similar. The browser would tunnel all requests through their server and render and optimize the page before sending it back to the browser