Apple has been playing around with both RAM and SSD storage over the last few years. · The 2014 models have soldered in RAM so what you bought is all you got (4, 8 or 16 GB)! But the storage was upgradable! It offered a SATA III (6.0 Gb/s) SATA port and a second storage option offering a PCIe/NVMe blade SSD. This allowed you three different configurations SATA drive alone, Blade SSD alone or a Fusion Drive which was both drives! Many people upgraded their systems with a fourth option dual independent drives setting up the blade as the boot drive and the SATA drive as their data drive (best setup in my option!) This is often a 512GB or larger SSD and a 1 or 2TB disk drive. · OK, jumping to the 2018 to 2020 Mac Mini’s · Things got turned around! Instead of soldering the RAM, Apple now soldered the storage! So you can upgrade the RAM unlike the older 2014 model, you are stuck with what you bought with in system storage (128, 256, 512 GB, 1, or 2 TB)! Remember this soldered in storage is also quite secure! As its driven by the T2 chip so its encrypted on the fly and also quite fast! · So what to do?? · You have the right idea of getting the limited RAM and upgrading it on your own. Here you’ll need to fork over more cash on getting as much storage (SSD) you can afford! I would recommend getting at least a 512GB, If you are composing/mixing music or video work then you might want to get a 1TB drive then make the investment into a good USB-C (Thunderbolt3) drive/s). Basically setting up a dual drive config having your internal drive your boot drive (OS), App’s and any sensitive info. Leaving as much of the drive empty as you can (1/4 to 1/3) so your OS & apps have elbow room for Caching, Virtual RAM, Paging and App Scratch space. Using your external for your media library and finished work. · While have 64 GB of RAM is possible, do you really need that much? I would recommend just getting two 16GB SO-DIMM’s pushing the system to 32GB or just get one 32GB SO-DIMM (plus one of the 4GB SO-DIMM’s will get you to 36GB) if you really think you’ll need 64GB at some point. · Futures! · As you likely know Apple is about to shake things up again! Next year Apple will be introducing systems with their own silicon CPU’s & GPU’s! Jumping from Intel CPU’s. · We have no idea what Apple will introduce first or even if these new systems will have more processing power as to what Apple now offers with the Intel CPU’s (with intergraded graphics) the 2018 ~ 2020 systems offer. · It’s likely Apple will rollout the replacement Mac Mini at the bitter end of the roll outs, but who knows! · Maybe Apple will rethink what a Mac Mini is! While my crystal ball is a bit foggy, I’m hoping they will offer a trimmed down Mac Pro offering, dropping the idea Apple started out with the mini as an entry system. · The current Mac Pro is massive and way too much for many to afford. Apple loves pushing the wall too far thinking thats what people will want. I’m sure the video studios loved it, photogs and graphics pros like me working on large images thought it was way too much! We need the processing, graphics, RAM and deep storage not all of the slots or fancy storage the current Pro offers. · In truth many people liked the 2013 Trash Can size but didn’t like the limitations it had! One blade SSD drive and a non standard GPU setup. · The reason I think Apple dragged their feet getting out the newer Mac Pro was the partly due to Intel’s CPU’s running so hot! So the cooling design would not work. Just look at how massive the cooling is in the current Mac Pro system! · Maybe with the Apple silicon we have a chance to see a smaller and cheaper option for a Mid tier desktop system which is not an all-in-one like the iMac’s and more powerful than what we have here in the current Mac Mini’s and offer three storage bays! One T2 the other two M.2 bays and still offer serviceable RAM. Give me a physically half sized Mac Pro and I’ll buy it!

Get a Samsung external SSD... don't play around with M2 and enclosures, the enclosures have been problematic in the past. Even a Samsung T7 is pretty good.. I have one. They also have the X5 NVME for more money.

You can find a guide on iFixIt, summarized on MacRumors.com: 2018 Mac mini Teardown: User-Upgradeable RAM, But Soldered Down CPU and Storage

Memory: Yes*

Apple specifically mentions on the Apple Website that the Memory are SO-DIMM sockets (2666MHz DDR4 SO-DIMM).

Mac mini features high-performance 2666MHz DDR4 SO-DIMM memory

*However, the warranty is voided if upgraded by the user as the modules are no longer directly accessible from the bottom but instead involve removing the antenna plate, fan and mainboard.

Storage: No

Storage is (like on the MacBooks) soldered onto the logic board and therefor non-replaceable (you would need to replace the entire logic board).