I believe Kindle can do this, if you use Send To Kindle and turn on the archive option. I used to do this with a Kindle Paperwhite and the Kindle app on my phone and computer, but it's been a while.
Ereader
For all things ereader
From what I've read, most ereader/app pairs only sync if you buy them from the respective store.
Yes that's always been the case when I have looked into this.
I think being on IOS might make this a problem. KOreader is the common recommendation to sync with kindle/kobo but I don't think it has an ios app unfortunalty.
From what I’ve read, most ereader/app pairs only sync if you buy them from the respective store.
They might have a feature to upload books to their cloud. These books would then sync between their devices and apps. For example, Kindle ecosystem has a feature called Send to Kindle that can do that. The easiest way to use it is to use the web app (https://www.amazon.com/sendtokindle) on computer: You just drag and drop files to that page, and they are added to your Amazon account.
I was hoping to avoid Amazon, but it seems like a kindle is the answer
Also do you know if the calibre send through email works for this?
I'm kind of surprised if Kindle is the only one that has that feature. Did you check the competitors?
You can use Send to Kindle email method to upload files as well. It works with Calibre. The page I linked has "Other ways to send" section which lists all the methods and how to use them. The web app has 200MB file size limit, the rest are limited to 50MB per file.
Also, while Send to Kindle supports EPUB, Kindle does not: Send to Kindle will convert the files to a Kindle's own file format. This conversion can sometimes mess things up, but they have been improving it since they added EPUB support last year. You might want to try out the Kindle iOS and Desktop apps before spending money on the actual device, and test how Send to Kindle works.
I was looking into kobo, but some reddit posts said that sideloaded books dont sync. It was a little older so I just wanted to see if anything had changed.
Thanks for all the information.