Hm. All you said is opposite of my experience with Kobo. I have a Forma bought in 2019 that does not lag, has great battery life (after almost 7 years) and all my ebooks work. I have some that I purchased directly from Kobo, some from ebooks.com and some from local stores (no DRM). The Kobo software always has been snappy, I always tell people that Kobo has quality build AND good software, unlike PocketBook (I had InkPad 3 Pro and Era) which lacks in that regard.
I agree that sync tends to be flaky between multiple devices.
I bought a Kobo Libra Colour a couple months ago because I wanted to try a color e-ink reader. Now I have two and I couldn't tell which one is better because each has some pros and cons over the other model. :)
I had multiple Kindles before switching to PocketBook and Kobo.
I wonder if your particular device had some issues or maybe it was a particular model. Forma was the high end Kobo device, now discontinued.