To anyone else who couldn't get the supplied screenprotector on without bubbles, I got one custom made instead at
https://www.protectionfilms24.com
It cost £8.45. I chose 'Brotect matte' with the dimensions 139x72mm and a 8mm corner radius. A little smaller than the supplied one, which made it easier to fit.
After cleaning the screen, I covered (and uncovered) it with no-residue scotch tape to make sure there were no tiny dust specs before applying.
No bubbles second time around, and performance seems good (matte finish but sharpness is still pretty good).
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It's my favourite reader, but here's my wish list in order if they ever make an improved version:
1. Micro-etched glass screen (like the Kindle Voyage had - great sharpness but matte)
2. Failing that a regular flush matte screen, or non-flush with the e-ink screen as the outer layer
3. Failing that, a factory applied matte screen protector
4. Better re-connection to Bluetooth devices after sleep (I often have to toggle Bluetooth to reconnect to my page turner)
5. Physical back button on the side instead of a capacitive one on the front (like the Hisense A5)
6. Silent page turn buttons (they're quiet, but still make a little more noise than ideal if you've got someone trying to sleep next to you)
7. Wireless charging (I don't want the weight of a bigger battery, but this would make regular charging slightly more convenient)
8. English language out of the box (I found it fairly easy to change, so low priority for me)
9. Even lighter (it's already very impressively light)
10. Even thinner