The base Mini is more than capable editing 1080p and exporting it super quick thanks to the T2 chip. The integrated graphics on the base mini (2018-2020) is very capable compared to the 2009 Macbook Pro which is based on the Nvidia 9400M (I have the 17", though it's now running Mint 19.3) so yeah, it's no wonder it is pretty excruciating to edit on. Having said that, video editing is NOT only about how fast your CPU is, but how capable your GPU is. The reason your Macbook Pro 2009 is so slow is because its CPU (Dual Core) is too weak to process the highbit rate off your iPhone XS and the action cam let alone the higher FPS (60fps and up). When you edit the video, the software like iMovie have to transcode your intraframe footage (compressed h.264 or h.265 files) into a set of interframe clips so you can edit them. After you finished editing the footage, you then export the edited project back to intraframe so you can play it back on your TV, your phone or your computer. This process is called "transcoding" and traditionally, it was done by the CPU. With the Mac Mini 2018, it is now done with a T2 security chip that is on the base Mini as well as the top end i7 model. With the T2 chip, the transcoding process is amazingly quick. What took my Mac Pro 5,1 to transcode a 1080p in 1hr, it will take a base Mini only less than 10 minutes for the same file!! So yes, the base Mini 2018 will easily smoke the 2012 Quad Core i7. I know, cause I tested them when I was working as IT for a Mac, PC and Linux recycling organization. Basically, your base Mini would be fine. If you want more capability, you should be focusing on getting an eGPU (external GPU) like the Akitio Node and marry it with a Radeon RX580 with 8Gb of ram. Basically the more video ram you have, the more fluid the editing on the time line it is for your video. So it does not matter if you get a Core i7 Mini 2018 or 2020 because it will be limited by its weak built-in iGPU which supports up to 1.5Gb of video ram. A base mini with a Radeon external GPU would easily edit any 4K video providing that you have fast USB-C external storage for your footages and finish products as 4K is 4 times the information of 1080p so file sizes and the speed necessary to read and write need to be fast as well. Hope this helps.