APPLE IMAC 19.1 (EARLY 2019) 5K

That model iMac has a fantastic display, but I'd opt for a M2 Pro Mac Mini or M2 Max Mac Studio instead.

Also, with some M4 based Mac models soon to be available, the M2 models should be available on clearance or refurbihsed at a lower price leaving something to go toward a nice display for it.

If you go with the 27-inch 2019 5K iMac, I'd consider booting from a higher capacity USBC SSD drive (2TB or 4TB) and using the internal 500GB Flash storage as a Media Cache/Scratch Disk location.  On the Intel based Macs, an alternate boot drive is very easy to configure.

About the specs for that early 2019 iMac, what is important are:

CPU, Intel Core i9: This is OK, although it might run hot when editing video. Also, it will probably be beat by any M1/M2/M3 Mac, because Intel Macs are generally much slower than Apple Silicon Macs like your M1 Mac mini.

Internal memory, 64GB. This is great, a nice large amount of RAM. However, this will not overcome the other weaknesses of this configuration.

Internal storage: 500GB SSD. At least it’s an SSD, but that is not very much space for video editing. External fast storage will be required to have enough room for captured clips, Media Cache, and rendered previews.

The graphics hardware is not listed there, and there are several Radeon and Radeon Pro options from 4GB to 8GB of video memory. It should be OK, but not great compared to the graphics you get in Macs today.

Overall this Intel-based iMac is probably not a good long-term purchase for video editing, considering how much more powerful the Apple Silicon Macs are.

If the only problem with your M1 Mac mini is that there is not enough internal space to save videos, then I completely agree with RjL190365 that you should attach fast external storage and configure your video apps to use it. If you are having other problems such as not enough unified memory (RAM), because that can’t be upgraded on your M1 then a good solution is to get a newer Mac mini that has at least 24GB of unified memory and 1TB of internal storage, but even with that you will probably have to connect fast external storage for video files that don’t absolutely have to be stored internally.

I say at least 24GB of memory because 8GB is much too low for today’s applications, and also because on top of what the system and apps need, for video editing it’s important to have enough free memory available for graphics acceleration. You do not want to be too close to the 8GB minimum.

Are you using an external USB 3.2 Gen 2x1 drive (especially an external SSD) at all? If not, that's the problem as you do not have enough internal storage available to save anything at all. You see, exported and rendered videos already take up tens, possibly hundreds, of GB just by themselves!