Internal SSD almost full

First, you should know that when a HD gets too full it will stop functioning and then you have a real problem. When you run out of space on your HD you will need to install an external disk. Then you can transfer files, such as photos or music files to the external disk and that will allow you to delete them from your HD to clear up the needed storage space. These days these disks are cost effective, compact, easy to install with vendor provided USB cables and most do not require external power. You may have to reformat it though. Use only Disk Utility to do that. There is a large selection of disks to choose from. This solution will allow you to easily have the storage you need.

Yes, you can add an external drive. Get a fast SSD if you can. You can put documents, music library and even applications on an external drive. I have my iTunes library, my documents and about 1/2 of my apps on an external OWC SSD drive. Works great. Perhaps out of superstition, I only put simpler apps on the external. Photoshop type apps stay on my internal.

Mac OS does create a fair number of files itself and they can start to clutter up a drive. Some here will disagree, but I run Onyx (freeware) once a year to dump temp and log files. It always frees up noticeable space.

Your external backup drive "shouldn't" have a significant effect on your internal storage drive. As Ronasara said, take care of this before your disk gets full!

You can always buy a larger external drive and clone your internal to that drive. You can then use that as your boot drive and keep all on that drive while using the internal for misc. stuff.

To be safe you need to leave at least 10% free or 25 GB in your case.

Open "About this Mac" from the Apple menu and search for "Storage Settings"

Your Mac will quickly list all the files on it starting with the largest and give you the opportunity to delete any you no longer need.

This is how it looks on Ventura and Sonoma. You would click on the "i" at the end of each category to reveal the various files.

Good thing you use Backblaze - but it will backup (not save space). And what you remove from the mac will eventually disappear there toi (depending on your plan) - it saved me multiyears of audio work which i had lost due to bad use of iCloud drive.

However, you indeed need an external drive. As long as you don’t place critical working files that need to be internal (such as ongoing movie or audio work), you’re perfect for storage out of the box. Make sure you also have it backed on Backblaze as you will need to indicate this in the settings.

Just don't add any more unnecessary data (especially videos) until the new drive arrives. In the meantime, delete as much as you can that you no longer need. With videos again being the number one culprit of space hogging data.

Unless your friend has the exact same Mac you do, you don't want them to install macOS on the new external drive as it will not have the correct drivers for your Mac. It then may, or may not be bootable.

You would instead attach the new drive, then start up your Mac in Recovery Mode so you can directly format and install an OS on the new drive yourself.

And even that won't be necessary as long as you leave yourself that meager 5 GB of free space. Then all you need to do is format the new drive as APFS and use it as a dumping ground to get large files off the main drive and onto the external.

Just don't download and attempt to install anything new and I think you will be alright for now.

I've used OnyX for many years, and clearing space isn't really what it's for. The only function it has for that is removing various cache data and log files. Unless your log files are unusually big, not much will change since the OS and apps will simply rebuild the caches you cleared.

Yes, it's true of any drive. The OS and apps need cache and scratch space to work with. If the drive gets too full, pretty much everything will halt.