Rated 5 out of 5 stars

It works perfectly although it is a bit complicated to setup.
Edit: The first thing I would simplify would be the main menu that pops up when you select the send later. I would like the shortcuts to be on top since those are the most used ones. I would also have a default shortcut for next day at 8 or 9 since that is what most people would use the send later fuctionality for (probably). I know that send in 15 minutes or 2 hours is useful too but the next morning is what is being used 99% of the time (Perhaps it is a me problem and I am being out of touch). I would also redesign the manual input of day and time because it does not work if a conflicting input is selected under it in the between hours and days. Probably splitting in 3 modes would be the most ergonomic design. 1 for shortcuts, 2 for recur, 3 for completely manual. If manual is selected it will go without checking what is in the between inputs. Also I found no fuctionality for the shortcuts "add to outbox" and "send now" from the default settings. Send now is the default of thunderbird and add to outbox is not really useful since it would stay there and probably would be forgotten.

I know that probably those things are small and almost everything can be customised how I want it but since you asked I answered xD Great job anyway and I really like your work. You are awsome to create something so useful and be so kind to also ask for info to improve it!

This review is for a previous version of the add-on (9.2.7). 

Thanks for the feedback. Any specific things you'd like to see simplified?
EDITED TO ADD: A lot of what you are suggesting here is personal preference, and I am doubtful that your assumptions about what other people use "99% of the time" are entirely accurate.
I am not sure what you mean when you say "it does not work if a conflicting input is selected under it in the between hours and days." but it sounds like you're saying that there's some sort of bug, so I'd like to explore that further. This isn't really a good forum for back-and-forth, so if you want to continue this discussion I'd recommend either emailing [email protected], or starting one or more issues or discussions in GitHub about the issues you see and/or changes you'd like to be made to the UI (I would not recommend grouping all of your suggestions into one message as you did above; you should create separate issues or discussions for each category of change.
As for whether it should be an issue or discussion, if you'd like to get input from other people about whether they agree with your proposed changes then a discussion would probably be the right thing to do.
You can do issues here:
https://github.com/Extended-Thunder/send-later/issues
or discussions here:
https://github.com/Extended-Thunder/send-later/discussions