Although you've likely already decided, I thought it might help others who run across this thread to post my recommendation (trying to help).
The 2019 Mini is tempting but a 2019 upgraded iMac is the better ROI. Why? In short, the iMac has a built in great quality 4K monitor and there will be less possible thermal throttling issues with the iMac case rather than the cramped Mac Mini case. (For those who are unfamiliar with thermal throttling that is when a computer runs too hot thus the speed of the computer can significantly drop to protect the components. Electronics need good airflow and to keep cool. It is never a good idea to pack a bunch of electronics that will run hot in a very tight box with very limited airflow (i.e., Mac Mini/MacBook Pro). Something has to give. Not only is there a chance the computer will run slower it will also die sooner than a computer that has good thermal dynamics. Baking electronics = shorter lifespan).
Since I get the sense a restricted budget is in play and your core requirement is to edit GoPro videos (video editing is CPU and GPU intensive = high heat), I definitley recommend an upgraded iMac. If budget is not an issue (up to the absurdly priced Mac Pro) then go with the highest level and maxed out iMac (not iMac Pro). Focus your budget in this order of importance (#1 & #2 is a close call): 1. CPU (i7 or above, NOT an i3, 6-core), 2. Memory (16GB is bare minimum, 32 is standard, more is better), 3. SSD (500GB is a good middle ground; 256 is too little, stay absolutely away from any internal serial ATA HDD because that is ancient and slow technology - hang your head in shame Apple for offering that!).
I priced out the middle level iMac (4K, 3.2GHz 6-core, i7, 16-32GB memory, 500GB SSD). Depending on the memory size, the price will range from $2,200-2,600 +tax. That's totally fair for a good computer. A 256GB SSD is too small for too many reasons to list here. You will definitley need to rely on good external HD storage (buy 1TB SSD storage commonly on sale and a $10 enclosure) for any iMac you buy because the best practice is to put as little strain as possible on the internal iMac HD. The iMac HD is a royal pain to replace because of glue strips and such. You want to avoid that route as long as possible. macOS and core files should be the only ones on the internal iMac HD, other files go to ideally an external SSD drive. Backups go to external HD but you can certainly use a large capacity 8TB drive. (I like Seagate rather than WD. WD will not stand behind their warranty and they are a royal pain to deal with if there is a problem.)
In comparison, a similar Mac Mini (not recommended) will cost $2,200 PLUS you have the cost of a good 4K monitor (another $500) + keyboard ($100), + mouse (good quality, $60-90).... GRAND TOTAL around $2,900! Besides a higher cost, this computer will likley not last as long because of electronics damage from constant high heat.
So, it's a no brainer, the iMac is the better value and ROI. A last comment to note is don't always trust the seemingly "budget" option because when you take a close look at the facts it is usually not the best value. A good example is buying storage (GB). Smaller GB sized hard drives are usually more expensive per GB than a higher capacity drive.
Hope this helps!