I've never had any email on custom address I used for oldest Kobo, or on any others since. Not from any source other than my own test emails
The only email I used to register a Kobo that gets Kobo emails is purchase confirmations of ebooks bought from Kobo, which I download to PC.
I don't need WiFi or Sync on any ereader.
If the Kobo mounts as storage via USB you don't even need the Kobo Desktop. Simply download and unzip from here
https://pgaskin.net/KoboStuff/kobofirmware.html
Put the unzipped files and folder into .kobo on the Kobo Aura, eject and it will update.
The PRS650 is OK; I have the PRS350, but only advantage is small size. The H2O Original, Libra, Nia, Libra2 and Sage are all far better and integrate better with Calibre.
I looked at PRS+, but couldn't see the point vs risk of messing up a decent PRS350.
I have the Boyue Likebook Mars 7.8" and I think it's junk compared to Kindle or Kobo. The only advantage, and the reason I got it, is to use the Android App for the Libraries here in Ireland. It's got no sane library / metadata interface like Sony / Kindle / Kobo. Maybe be more use when KOReader gets a Metadata interface instead of folder browsing + Search. But MTP is a pain.