How to install Ubuntu on a spanned volume

I have been using Ubuntu for quite a while but am running low on space as I only allocated 50GB.
As a workaround, for now, I have mounted another drive in which I have stored some contents. I have installed Ubuntu on SSD and the drive I have mounted is an HDD.
I have mounted the drive as /mnt.
I wish to reinstall Ubuntu but this time I would like to install it in some sort of spanned drive. I would like / to consist of both SSD and HDD
Any solution is accepted as long as I don't out of space in the future because now I have to store some things in /mnt and some in /home, can't there be a way in which I can use both in the same way.
Can somebody guide me on how to do so?

Merging volumes to install an operating system will be highly complicated and make the system more vulnerable. Your current setup is fine. Storing data on another disk is not at all a workaround. It is a perfectly viable solution to expand space for data storage.

You can very efficiently and easily organize data storage in Linux using symbolic links. Thus, you can easily and seamlessly access any data from within your home directory, even if it is stored on another partition.

Suppose you have a "Music" folder taking a lot of the 50GB. Move that folder to the /mnt directory on the other drive. Now create a symbolic link "Music" in your home directory that points to the real Music directory under mnt: now you can access your Music from within your home directory just like before.

To create a symbolic link:

Method 1: using Files

Open two windows of Files, one open where your moved "Music" folder is, and one open in your home directory, where you want to create the link.
Hold Shift+Alt. Drag the Music folder to the other pane and release the mouse button: a symbolic link "Music" will be created that, for all daily uses, acts and feels like a real directory.

Method 2: terminal

Use the ln -s command to create symbolic links, e.g the command to create a symbolic link "Music" in your home directory that refers to the real folder "Music" on the mounted disk.