the username change will solve 0 problems and create way more

"people can't remember their discriminator"
-skill issue.
-i work retail, lots of people also don't know their own phone numbers or zip codes; designing around people's inability and/or unwillingness to retain important information is a losing battle. if people actually can't remember their discriminator, they can look at their phone! 99% of people who use discord are probably carrying a phone!

"you can't verbally communicate case sensitivity easily"
-skill issue.
-pencil. pen. text each other. add each other on phones right there.
-yes this one is slightly more of maybe almost a real problem but you made your bed with this one and you need to lie in it instead of creating more problems lmao. people deal with shit like wi-fi passwords with case sensitivity all the time! you just kinda... do it! you just try!

"there are already 9999 mikes"
what you're proposing lowers the number of possible mikes to 1.

"you like to change your username a lot and get rate limited"
USERNAME CHANGES GEORG IS AN OUTLIER ADN SHOULD NOT BE DESIGNED AROUND, SORRY.

"(the issue with special character usernames)"
okay, well, again, YOU MADE THIS BED, and if Special Characters Liker's friends have trouble adding them, they already have the choice to change THEIR username to be easier to share. :| it's literally on them.

P.S. for every exchange of 2 usernames, there are TWO PEOPLE. it is a two-way street. each of them is capable of requesting to add the other! if one of them has a Username Of Extreme Difficulty, then the solution is for the person with the Difficult Username (or the person who doesn't remember their discriminator) to say "oh give me yours and let me add you, mine is weird".

P.P.S. remembering your discriminator is something that you could reinforce with users if you tried literally at all. you could like... add a little game into the UI somewhere that asks you to try to remember it and lets you use a fancy avatar frame or something for the month. like a change-your-password-it's-old prompt except "hey let's see if you remember your four numbers :)"!

sorry but the problems you are trying to solve are not real or dire enough to justify the absolute pandemonium you are about to create.