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

No, and this catches people out. 2Do's backups are stored inside the app's own container on your device. They are not uploaded anywhere on their own. Delete 2Do, or erase the device, and every backup inside the app goes with it.

The fix is to keep a copy outside the app. Open Settings > Backups & Export and either:

• Tap Backup to Email to create a fresh backup and send it to yourself, save it to Files, or AirDrop it, or
• Open the menu beside any stored backup and choose Save a Copy… to put that file in Files or iCloud Drive.

Make it a habit. Weekly is reasonable, plus one extra copy before any big change such as an erase, a device replacement, or a switch of sync service.

To use one later, open Settings > Backups & Export > Restore from File… on the new device and pick the file.

See Backups for the full guidance.