Copy individual files onto TimeMachine backup

When making a drive a Time Machine drive it becomes owned by the Time Machine App and your access to it will be Read Only. Therefore you cannot alter the contents of the Time Machine, only macOS can do that.

Older advice may not apply in this scenario. Time Machine now uses the APFS file system and it's less risky to work with than older file systems. You can actually add an APFS Volume non-destructively and it won't even require being sized. APFS Volumes in the same container share the same free space of all volumes in the container. Just keep in mind you are reducing the total capacity of the Time Machine volume as you add data to the second APFS Volume.

Open Disk Utility and click the View menu and Show All Devices. Locate the Samsung drive on the left-sidebar and highlight the Container on that drive. Right-click and choose Add APFS Volume.

When the disk is attached, you should see two disks mount, each one is an APFS Volume. One of them will be your Time Machine drive that is always Read Only to you. The other new disk is your data drive that you can Read/Write whatever you wish.

Yes, less space available for Time Machine backups. I typically recommend two to three or times the capacity of internal storage for a Time Machine backup drive. You need enough space for the full copy of the internal disk plus snapshots. The larger the Time Machine drive the further back in time you can go.

APFS volumes within the same container share the free space between them. When you add files to the second APFS Volume you are reducing the total capacity of the 1TB disk as it also contains your backups on the Time Machine APFS Volume. So long as you don't store too much data on that extra volume you will be fine.

APFS is not compatible with other operating systems. It only works on macOS. So plugging this 1TB drive into a Windows or Linux computer, the disk will be unrecognized and unreadable.

If your Time Machine backup fills up every month or two then you need a larger external drive. If the 1TB SSD drive is your Time Machine drive, you should buy a larger capacity external drive to use for backups. Typically recommend at least two to three times the size of the internal disk. The larger capacity allows for longer term history of changes.

SSD is very fast but also very expensive. Perhaps too expensive for a backup drive. An external HDD will work fine, offer far more storage capacity and it won't be working all that hard when it backs up once and hour. Perhaps a 6TB or 8TB HDD would be a good choice. As to what brand, so long as it's a mainstream brand you are good. WD, Seagate, LaCie, Toshiba, etc. The rest is dependent on your budgetary constraints.