Use of portable hard drive

Yes you can, but it is a bad idea because you will lose your photos and backups if the external drive ever goes belly up. A better idea is to uses two external drives, one for your Photos Library and one for backups.

Yes, you can do this. But you should let Time Machine have its own space where it will be undisturbed and you won't likely do some "accidental damage" to the backups.backupdb folder or the files within.

Perhaps the best way is to partition your new large drive as two volumes, one for you data and one for TM to use. Give TM plenty of room to grow, maybe 2-3TB, and se the rest for your data.

You can partition the drive using Disk Utility.

Then open System Preferences > Time Machine and configure TM to use the partition that you created for it.

True. 👍🏽

Good plan, you're welcome!