when trying to set the local-part for my custom domain email address to "~"
I get this error message:

"Only alphanumerics, periods, hyphens, and underscores are allowed in the local part of the email address, and it must begin and end with alphanumerics." all I can say is why?

"it must begin and end with alphanumerics."
this is so arbitrary! it's an email address' local-part not a username..

for many years, I have always used ~@myna.me as my email address, before and after I self-hosted my email, I use it for all of my online accounts and and all of my contacts have it, I can't change it now.

this is a spec compliant email address local-part, you don't have to read any of the specs, just read this Wikipedia article section:
https://en.wikipedia.org/wiki/Email_address#Local-part

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I have another email service that allows users to schedule draft emails to be sent at a specified date/time in the future. Once the draft is complete, the email is automatically sent at the future specified date/time, unless that draft email is deleted/replaced prior to the send date/time. The draft email can either reside in an existing "draft" folder or a separate "outbox" folder (as distinguished from "draft" that does not have a scheduled send date/time).

Different Emphasis from Similar Postings:
I noticed a number of other similar postings, however, I wanted to provide this new posting to possibly emphasize something more than what I saw emphasized in other similar postings: closely related general delivery options. I would suggest ProtonMail allow every draft email to have a drop down menu that provides the option for selecting "Delivery Timing" using a calendar display for selection of a date and a time field to select/enter a corresponding time on that selected date (time should be clearly set for a selected time zone, e.g., CST or GMT, to reduce possibilities for confusion on timing). Again, whenever "Delivery Timing" options are selected, the draft email can then reside either in the existing "draft" folder or in a newly appearing "Outbox" for any pending emails to be sent at a future date/time. The other email service I use provides a range of closely related general delivery options to also include:
-- "Message Priority" (Low, ******, High);
-- "Private" (to help prevent email forwarding, if clients support that);
-- "Return Receipt" and
-- "Delivery Timing" (previously addressed)
I recommend ProtonMail make these general delivery option menus accessible via one additional icon next to the existing row of icons on draft emails for "Attachments," "Encryption" and "Encryption time." The icon could be a clock next to a letter (just a suggestion), since "Encryption time" already uses a clock icon.

I don't believe other similar postings emphasize these closely associated general delivery Options. I have previously given this other email provider feedback on perfecting their services in other areas (with different providers having different strengths/weaknesses), but this is one particular feature they have had for a very long time that I believe provides an excellent example to follow.