It's been a while... but the To2Do apps are being updated to work with Big Sur :)
The first app (MailTo2Do) has been updated and is available here:
https://bit.ly/mailto2do-v210
The other apps will be updated soon.
If you'd like to donate towards my running costs of this site and the maintenance of the apps, please go here: https://bit.ly/jgibbard-donate