Ditch USB-A and replace with a pair of USB-Cs (on the back) with a thunderbolt connector for faster NVME SSD speeds and I'd pay 3X what they are asking.
Intriguing thought, but I know of no product quite like that - at least that sits under the M4 series Mac Mini and matches its appearance.
The closest to what you describe off the top of my head is the
Sonnet Echo 20 Thunderbolt 4 SuperDock. At around $300 it hits your price point, offers a bunch of posts and has an internal M.2 NVMe SSD slot. It would not sit under the Mac Mini or match its appearance, but it would be drastically more functional than what you describe except in one key issue (and this is where one must read the fine print)...the SSD speed is only up to 800 MB/s (per the Amazon description I linked).
Given that the cost of an external SSD tends to not be drastically greater than an internal SSD (though with Thunderbolt enclosures that goes up), it'd probably make more sense to buy an external Thunderbolt 3 SSD and plug it into your Mac Mini's rear Thunderbolt port, or one on the SuperDock.
From other threads, I've gotten the impression Thunderbolt external SSDs give off a lot of heat (from the controller and the SSD itself operating at high speed, IIRC?). Packing all that inside a dock that has a lot of connectors in a compact space may not be practical; sticking it under your Mac Mini where it's sucking in air to cool itself might be even worse.
All that said, the SuperDock would offer far more ports than what you describe and so a lot of potential extra function. If you wanted to save some money and didn't need so many ports, Amazon has a sale on the
UGREEN Revodok Max 208 Thunderbolt 4 Dock 8-in-1 40Gbps Thunderbolt 4 Hub 3 x Thunderbolt 4 Dual 4K@60Hz or Single 8K Display, 85W Charging, Gigabit Ethernet, 3 x USB A 3.2 for Mac M1/M2/M3/M4 Pro/Max
for roughly $250 marked down to $160. Less money than you offered to pay, you get 3 Thunderbolt 4 ports (net gain of 2) + more ports, and no internal SSD slot granted.
Does anyone here know of a Thunderbolt dock with an internal SSD slot that supports 'real' Thunderbolt speeds, like 2,800 Mbps and up? Is it just not practical from an engineering standpoint?
I like the Satechi, it's cute, but I'm having a hard time thinking it's a good value proposition. How much extra money is having something sit under your Mini and match the style worth it to you guys?