Looking to purchase a Mac Mini, a few questions about upgradability/monitor support/IO

I purchased a 2012 Macbook Pro (non-retina) last week and before I even turned it on to play with it, I maxed out the RAM and put in an SSD. For being a 6 year old laptop, I'm blown away. I've never used a Mac (aside from my iPhones which I've been using since Gen 1), but I'm absolutely in love with macOS and I often find myself sitting at my desk with the MBP instead of using my custom gaming PC for internet browsing and such just because I appreciate macOS that much.

So I've been tossing around the idea of picking up a Mac for a desktop setting. An iMac sounds nice, but I already have 2 quality monitors and don't have space for a third. So I thought a Mini would be nice to use.

But I have some compatibility questions between both the 2012 & 2014 model. I mention those two only because I need Mojave support for the dark theme. I've got a sensitivity to bright white light, and have to run Flux/Dark themes 100% of the time.

Because I am a lifelong PC user, I appreciate being able to open and replace components as needed. I understand not being able to upgrade the CPU, but I would absolutely need to add an SSD and max out RAM. So I'm looking for a model that will allow me to easily replace both of those components.

Current monitor support. Both of my monitors are 1440p (2k, QHD). Not being able to use both monitors at once isn't a dealbreaker, but it's something I would like to have. They have Displayport & HDMI inputs. From the pictures of Mac Minis I've seen, it only seems like they have 1 hdmi port. I'm not entirely sure if HDMI supports 1440p, but I wouldn't mind having to downscale to 1080p. I'm not expecting 144hz, but having 60hz is a must. If I'm able to to drive both monitors, how would that work as far as I/O is concerned? Since my PC is using the displayports, I'd like to stick with HDMI so I could easily swap between inputs via the monitors OSD if needed.

Lastly I'd like to have a wired Keyboard/mouse (Could I use my current ones?), not a fan of battery/wireless devices. Otherwise do they sell wired Apple Mice, Keyboards, Trackpad (I fucking love the gestures the MBP trackpad offers)?

I'm not expecting this to be a powerhorse, I'd only do extremely light gaming on it (if that). I just want a proper desktop version of macOS