Yes, I agree with you. The internal SSD soldered inside the 2018 Mac Mini kills my desire to buy it. I knew you can use TB3 external SSD to run OSX. But a dead SSD might stop the system from boot (Happed on my Asus H170i-Pro/CSM with a dead SATA M.2 SSD) then you have to ask Apple to fix it.
Two issues with SFX motherboard. The first, their backplate I/O ports are too high to fit into Mac mini. The second, they use H110 or H310 chipset that only support PCIe x2. I replaced my H110T/CSM with H310T/CSM and put a i3 8100T that has a HD 630 like the 2018 Mac mini. With H310T/CSM, I can install a NVME M.2 2280 SSD but the max read/write speeds are around 1500MB/sec at most.
Here are the pictures of my latest mod. Got a metal frame and plastic back plate with opening. I used CR-10 to print a I/O backplate. At the same time, I got a mesh bottom cover from the same vendor in China.