Are my iPhone / iPad backups safe if I delete the 2Do app?

No — and this catches people out. On iOS, 2Do's backups are stored inside the app's secure container. They are not automatically uploaded to iCloud or any cloud service. If you delete 2Do (or erase your device), every backup stored in the container is wiped along with it. The fix is to keep an off-device copy. Open Settings > Backups > Backup to Email and email the file to yourself, save it to Files / iCloud Drive, or AirDrop it to your Mac. Make this a regular habit — weekly is reasonable, plus an extra share before any major device change (erase, replace, OS upgrade). See Backups (iOS) for the full guidance.