Suggestions (and a problem)

A) Combine “On timer” and “On referrer”

Both are triggered on pageload
The only difference is the check afterwards: delay or referrer.
And referrer misses the delay option. => Could be added, but then you have an “On timer” with delay and an “On referrer” with referrer and delay.
So, my suggestion is the combination of both options as “On pageload” with delay and referrer as additional settable options.

B) Ad an automatic closing option to “On hover” (preserving movable pointer) like in your own linked explanation: https://www.w3.org/WAI/WCAG21/Understanding/content-on-hover-or-focus.html

Example: https://greenshiftwp.com/sliding-panels/#block-sliding-panel

This would open new options like in above example and also classical “Tooltips” – especially in combination with “attached”.

C) “Attached” would be much better with optional selection of position “Above”, “Left”, “Below”, “Right”

D) Problem: When “Attached” selector is the same as trigger selector (Popup is attached to Trigger itself + show each time on click or on hover), the opened popup could not be closed with integrated close button.

E) Add hint in plugin (with link to detailed explanation) of cookie usage “mie-opened” + “mie-last-written” when using “Show again after days”.

At least in Europe is in times of GDPR any Cookie usage a very sensitive area. Users should know about in advance and in every detail, because they must declare it afterwards by themself in their websites in declaration of used cookies. (Also non personal, systemic cookies must be declared!)