Currently I have a Mini M4 with 16GB RAM and 512GB SSD. I am flip flopping daily between the Mini (various configurations) and a MacBook Pro 16”.
Your M4 Mini is pretty up to date, and you already have a 512GB SSD, so the debate over the 256GB base model is moot. For the moment, I’d say that 16 GB RAM and 512GB SSD are in the “if you need to ask, you don’t need any more” category.
The question is, then, what
isn’t your Mini currently doing that you need to do? You know how much of your SSD is free, you can check the RAM pressure while you are working and you know how fast the applications you use daily run. All I would say is that you don’t want your system drive to get anywhere close to 100% full - but on a Mini with 512GB it’s no great hardship to move your bulky documents, media libraries etc. to external drives and keep the super-fast internal drive for system, apps and temporary files/swap. (256GB gets a bit tight if you have a couple of pro apps & their libraries, large games or virtual machines installed).
There’s still quite a jump between M4 and M4 Pro minis with the same RAM/SSD capacities - so if what you decide you need is 32GB RAM and/or 1TB SSD you’ll still save about $400 by sticking wit the M4. The more sticky point is the top-end M4 Pro vs. the Studio M2 Max - but if you’re at that point I’d just wait and hope that a M4 Max Studio appears soon.
As for MacBook Pro - again, the simple question is, do you need to move your computer around a lot or use it “on the road”? If so, post Apple Silicon - with both desktop and laptop ranges using the same SoCs and giving comparable performance, then moving to a laptop might make sense. The only equation that really changes is storage - you probably don’t want your laptop to rely on an external SSD.
I think it’s unlikely that Apple will release another 17” laptop - the 16” is already a step forward after years of maxing out at 15” - and the switch to retina displays is a game changer. The 16” can cram a lot more information on its screen than the 17” ever could, so it comes down to what your eyesight can comfortably deal with. Your Eyeballs May Vary. Probably worth having a play in a store and trying out the various screen modes.
The way to beat “Apple’s up selling strategy” is to be clear about what you actually need and not get distracted by FOMO or a craving for “new computer smell”.