Yes, I am afraid that is the way to go, please consider a method on the linked thread, How do I upgrade from Android version 9 to a higher major version? - #2 by aibd.
- (Feel free to continue there with further issues as you may get better visibility to your upgrade issue than on this thread.)
Many guides advocate a whole system backup, so this becomes extra confusing. I prefer to have several means of backup. If you use Mumena ecloud successfully, then quite a lot is already backed up. (That is to say that the things you see from your PC are already backed up there.)
The support topic guide https://doc.e.foundation/support-topics/backup-and-restore-ephone takes as its starting position the things you may regard as most important.
As you will be potentially making a big jump in Android version from Android 9 (P) to Android 13 (T), it might be useful to install List My Apps, de.onyxbits.listmyapps. Save to output somewhere easily read and edited like your PC. Now be harsh in preparing to uninstall all apps where you really could do without their data after the Factory reset. Also check if any apps you need offer an option to “Export data”. I feel the tangibility of an Exported file a most useful form of backup.
Sorry I am not a Whatsapp user, but I think this may be your hardest bit! I include this link, while quite old I hope it describes (as I understand it) the permission issue you will face and the apparent need to have your restored material on your new system, before reinstalling Whatsapp and logging in. https://android.stackexchange.com/questions/150071/how-to-manually-transfer-entire-whatsapp-installation-to-another-device-the-har .
There is this [HOWTO] Transfer WhatsApp messages from Google Android to /e/ (backup and restore)
And of course Search forum for backup.
If you do use Nextcloud there is this method Whatsapp backup - #11 by daan-banaan. As a “backup alternative” you might experiment with https://itsfoss.com/whatsapp-linux-desktop/.