Expansion of G-Technology 4TB Thunderbolt Drive Capacity

I’m still fairly new to Mac and RAID technology, so pls bear with me.
My problem is that I’m running out of backup space, and I want to understand my options for solving the problem.

I use a 4TB G-Technology external drive for Time Machine with my iMac, connected via Thunderbolt. This drive does not allow daisy chaining - I just have a single Thunderbolt port and one USB 3.

I assume that Time Machine needs to see a single logical drive. I’m also sceptical about buying another G-Technology Thunderbolt external drive (also 4TB), and configuring a RAID array with the two of them, and get an 8 TB logical drive. Am I right in thinking this?

I have one free Thunderbolt port. If I were to buy a 2nd drive and connect it there, could I use RAID to configure a single 8TB drive?

Thx for any help.