Since you are running Thunderbird on two Windows PC the best way to synchronize accounts, folders and messages on both PC and the server is to use the IMAP protocol on both PC. This would also automatically solve your additional question about combining the Sent folders.

As I understand it, with IMAP if I delete messages from the server, I also delete them from my computer. Having my old trash is more important than having synchronization. Many time I have found things in trash on the computer that I never thought I'd need again but did, &/or wasn't supposed to have been deleted in the 1st place. My goal is to be an elephant & never forget!

As I understand it, with IMAP if I delete messages from the server, I also delete them from my computer.

Correct.

Having my old trash is more important than having synchronization.

This is your decision. Trash is trash and no archive. To change from IMAP to POP https://support.mozilla.org/en-US/kb/changing-imap-pop

Are you sure that link is current, it's nearly a decade old. Also, only 50% of users found it useful. If it is still current, is there a way to migrate the sent emails from the IMAP acct to the new POP account?

Google is your friend: https://www.skeltron.com/faq-changing-imap-to-pop/ Set up the new account as a POP account. When you are ready the account will appear in the folder pane in parallel to the IMAP account. Copy all messages from the IMAP folders to the correspondent folders in the POP account. If everything is OK you can delete the IMAP account via Account Settings

Maybe I asked the wrong question. I want the entire POP Tbird acct from the old W10 computer on the W11 computer. The links above appear to just set up an empty POP Tbird acct. Somehow, months ago I got most of the inbox emails into the IMAP W11 Tbird, but I now see many are missing, but still on the POP Tbird W10 computer.

Are you sure that link is current, it's nearly a decade old. Also, only 50% of users found it useful. If it is still current, is there a way to migrate the sent emails from the IMAP acct to the new POP account?

Of course folk said it didn't help. It told them they could not convert their account the needed to remove it and add it again. You are still looking for the same information more than a decade later. You can't change an account type once the account has been added. Until that changes (probably never) then the information will remain "current"

Maybe I asked the wrong question. I want the entire POP Tbird acct from the old W10 computer on the W11 computer. The links above appear to just set up an empty POP Tbird acct. Somehow, months ago I got most of the inbox emails into the IMAP W11 Tbird, but I now see many are missing, but still on the POP Tbird W10 computer.

See https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

Thank you Matt, I was referring mostly to the directions in the link. 10 yrs. in computer time seems like generations ago, could have accounted for some of the 50%, but apparently not. Tried the 'moving Tbird data to new computer' link. The Profiles folder is 14.6 GB which in theory should fit easily on a 64 GB flash drive, but while copying goes haywire @ about 10 GB & says "The file 'Inbox' is too large for the destination file system" & stops!?! There are several inboxes from several different ISP's & addresses over the yrs, so don't know which one. Now what do I do?

To switch your Thunderbird setup on the Windows 11 laptop from IMAP to POP3 without losing any emails, follow these steps:

Backup Your Emails: Before making any changes, it's always a good idea to backup your emails to avoid data loss. You can either copy them to local folders or export them.

Delete the IMAP Account in Thunderbird:

Open Thunderbird on your Windows 11 laptop. Go to Tools > Account Settings. Under the Account Actions menu at the bottom, select Remove Account for the IMAP account. Make sure you're removing just the account from Thunderbird, not the emails themselves, which should be saved on the server. Recreate the Account as POP3:

In the Account Settings window, click Account Actions and select Add Mail Account. Follow the setup prompts and select POP3 when configuring the incoming server settings (instead of IMAP). During the setup, ensure you select the option to leave messages on the server if you want to keep them available on your provider's server for backup. Download the Emails: Once you’ve reconfigured the account to use POP3, Thunderbird will download the emails from the server to your local machine.

As for your second question about combining sent emails from different computers:

Use IMAP for Sent Items Syncing: Since IMAP syncs all folders across devices, the sent emails should appear across your devices. To combine your sent emails from both machines, move or copy the sent messages from your POP3 account on the old laptop to the Sent folder on your IMAP account. Manual Solution: If you can't use IMAP, you can manually copy the sent emails from one machine to the other by exporting and importing emails using Thunderbird's export/import tools, or by simply forwarding them to yourself.