I also faced the same issue for so many years. and nothing found on the internet. But after so a long time, I finally got a solution for this. I found a little but excellent working add-on for this, the link I am giving below.

It's very easy, we have to just install the addon, then copy Excel data from Excel, then go to the Thunderbird compose window, and just press key combination CTRL + Q, and you are done.

No need for MS Word or any other kind of word processor. you data will be pasted as it is with rich text formating with colors also.

https://addons.thunderbird.net/en-US/thunderbird/addon/paste-excel-table-into-compose/