The share icons can't be rearranged manually but there's a trick to pin your favourite targets. After you select share, long press one of the apps listed and you should get an option to pin it. Well, at least that how it works on Pie.
In your case the catch is that your favourite target doesn't show up on the list. Which app is it? And from which apps are you trying to share? Have you tried from Google apps like Chrome, Docs, etc?
It's possible that her email address gets back to share menu after sharing a few more links with it. Then remember to pin it to keep it in that menu.The app is an email address (wife) who is interested in articles from my newsreaders. A few days ago, SHARE worked perfectly in that her email address was in the list. If the target wasn't in the list, I would use the account NEW EMAIL, manually entered the address, and subsequently would appear the next time I used the SHARE command. Trying the suggestion of using Chrome etc didn't make any difference. I'm now using the workaround but its a pain in the butt. Thanks for suggestions.
It's possible that her email address gets back to share menu after sharing a few more links with it. Then remember to pin it to keep it in that menu.
And yes, the share menu is a big mess.