Read-Only Permission / Time Machine

Use One drive for several purposes including Time Machine Back can be done but strongly suggest not to do this.

Time Machine Backup whats to take 100% Control of the Entire Drive and will make the drive a Read Only - Normal

Added / multi purposing one drive is risky IMHO.

All the Eggs in One Packets and it fails ( they do and often times without any warning ) now Everything is lost and Scrambles eggs is all that remains.

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful.

For addition purposes - two Tested Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

While I've never tried it, my guess is that Time Machine doesn't allow file write access to other software. USB drives are so cheap, I only use the drive for Time Machine. It would be logical to assume that Time Machine restricts write operations to ensure that your backups do not become corrupted by possibly overwriting or deleting files by other software.

Personally, I would never use any cloud-based storage for backup unless you have a local copy somewhere. I believe that you need to be able to touch your backup and not rely on the whims of other providers. I backup to a NAS drive with RAID which gives redundant backup. Then once a week, the NAS drive automatically backs up to an external USB drive so technically I have three copies of my important files. USB drives have gotten so cheap it is not expensive to have two or more backups that you rotate between.

on my side I've a HDD dedicated for TM, so I do not know if all the HDD should be dedicated to TM or if you can create a partition to TM and the rest of the HDD for others purposes

IMHO, having TM as read only doesn't matter, as you should dedicate a separate drive to TM, and nothing else.