Have you thought of doing it directly server to server via webmail? https://support.microsoft.com/en-gb/office/turn-on-automatic-forwarding-in-outlook-7f2670a1-7fff-4475-8a3c-5822d63b0c8e#picktab=web

Then your gmail imap account would synch with server and download everything.

In Thunderbird When you right click on highlighted emails and use 'Copy-to' gmail Inbox, that is uploading to gmail server via imap.

Gmail have limits on uploading via imap. https://support.google.com/a/answer/1071518?hl=en

If you have an Anti-Virus scanning everything then it can take forever as it scans everything being downloaded/uploaded and also every file that gets opened. Sometimes things seem to drop out. Hence advise on using 'Copy -to' and not 'Move-to' as that second option may delete emails off the original server - if something goes wrong and you use 'Copy-to' at least you can repeate the action. Suggestion - Access Anti-Virus and set up an exclusion to not scan anything in your Thunderbird profile folder.

Thank you. I'm pretty sure I don't know what I'm doing. I have switched OUTLOOK account to default, quit trying to make Outlook.live.com sent emails to Gmail.google.com, and letting TB just download Outlook and Gmail emails directly. Attempting to let TB move the Gmail files to the Outlook folders. Is that the best way? My "old" way, I started missing emails from certain people and institutions. Don't know what went wrong. Worked for a long time. Outlook/Gmail must have changed something.