I looked at several alternatives before settling on GrapheneOS. Many made claims that sounded good but only GrapheneOS had deep technical documentation. I'm not an Android developer so some of it goes over my head but I've written detailed software documentation. I know you don't write what is in their FAQ or revision history if you don't know what you are doing.
I've been using GrapheneOS for about a year and it has worked well. Only have two minor issues:
1) I'd like the ability to have timestamp data added to the EXIF image info. There is an option to have the GPS data added, so timestamp really ought to be an option too, though I certainly understand why some users would choose neither. I'll probably end up writing a Perl driver for exiftool to add the timestamp based on the image filename which is YYYYMMDD_HH_MMSS_MSEC.
2) There aren't many fonts and Roboto supplies 1037 glyphs. The card suit symbols are missing which matters to a bridge player. E-books that display correctly almost everywhere don't on GrapheneOS unless the e-book explicitly includes a font with glyphs for the suits.