HERE IS HELP:
Since Thunderbird 68 - You can use your Stationery files with SmartTemplate4. Simply open SmartTemplate settings and select "File Templates" from the Accounts dropdown; you can then add your Stationery files to the write, reply and forward buttons. I have been using Stationery for years and worked with the author Arivald to make both our Add-ons compatible with each other.
Since he has no time to work for nothing for the many weeks that it takes to update Addons to the latest Tb version I decided to incorporate the Stationery feature into my own Add-on, because external templates are absolutely crucial for a productive Mail environment. Stationery has always been a 5 star Add-on, absolutely important.
I am programming Add-ons for 10 years and I would have patched Stationery if I had a month of free time (I seriously tried). But I had to set my priorities and look after my own Add-ons.
Full disclosure: SmartTemplate4 requires a license (5$ / year) because it is impossible to work on Add-ons every day of the week for nothing. All "free as in beer" things will end eventually, because they exploit people.