I would recommend purchasing a kit of 2x8GB DDR4 RAM (2133mhz or 2400mhz), replacing your existing RAM entirely. If you wanted to go to 32GB, you could purchase a kit of 2x16GB DDR4 RAM as well, although 16GB is more than enough for the typical user. Then just sell your existing RAM kit (2x4GB) locally, online, or save them for another system. The 2x8GB kit ...
A. Gigabyte Tech Support asks you to contact them to obtain the Z390AORUSPROGA9.zip BIOS and apply it to your existing motherboard. See https://pastebin.com/uqYt3rmx for details of their response. I regret I can't provide a direct link to the BIOS update file they suggested, so you have to contact them to get it.
B. Gigabyte TS replied and suggested their ...
Pick from this list
As @peterh mentioned, 970 and 990 boards (generally) support 64GB of RAM (ignore the ECS boards that list 128 GB, that's incorrect.) Since these boards aren't widely available anymore (due to their age and relatively limited popularity), a specific recommendation is pretty much useless as it's entirely dependent on what you can find on ...
Here are kit form suggestions for you:
The high performance Ryzen APU of the ASRock DeskMini A300 single board computer provides 60Hz vertical resolution in 4K video from its HDMI 2.0 video output and will use up to 3200 MHz DDR4 RAM, thereby meeting your requirements 1 and 2. Here are two reviews.
Intel's Extreme NUC 9 SBC-chassis-CPU kits have similar ...
I've used the ASUS B450-F for my last Ubuntu build. It was originally a R7 1700x on an ASUS B350-F, then I later swapped it for a R7 2700x on an ASUS B450-F. No issues with either board.
Unfortunately it does not appear that your board can support a TB3 add-in card. The reason for this is because it does not have a THB_C port.
MSI X99S SLI PLUS Specifications
Here are some examples of add-in cards that are compatible with motherboards that have a THB_C port:
Gigabyte GC-Alpine Ridge
Asrock Thunderbolt 3 AIC
ASUS ThunderboltEX 3
After our discussion in comments and looking at your requirements - you probably want to go with the nVidia Tegra. As a SoM not as a bare SoC. That's roughly the chip which powers the Nintendo Switch. They did port the Witcher onto it.
Any B450, X470, or X570 motherboard will work with your Ryzen 5 2600. In addition, you could flash the BIOS of a B350 or X370 board, although getting one of the first three chipsets will mean out-of-box compatibility. I personally like ASUS boards, although Asrock, Gigabyte, and MSI tend to make decent boards as well.
Here's a couple recommendations off of ...