Under $1000
4 channels
10/12 bits
differential input (or maybe just isolated ground)
100 MHz front end, 500 MS/sec on each channel
1 Mpts storage
This is very close to the Keysight DSOX1204A except for the 10/12 bits and the differential input
One option is R&S® Scope Rider RTH1014 - that will be 5000 €.
Isolated input scopes are mostly handheld these days, for specialised industrial electronics uses in the field.
I believe there is one Tektronix benchtop (TPS2000 2.5kpoints 3000-5000€), and there is a Cleverscope that is a 14bit USB beast that will cost you 9600 USD.
You have wrong perception as how hard is to make some things and how much it is supposed to cost.
Differential inputs are handled by using differential probes on a plain vanilla scope.
More than 8 bit scopes are specialistic instruments. They are used in power electronics for instance and such (like Mosfet switching analysis for efficiency calculations and such) . Not really needed for general purpose work. So they are not mainstream, so sales volume is low, so no economy of scale. And also, it is also awkward to move around 10 bit words, to do math on 10 bit words etc.. Tricks are needed, usally just using 16Bit word width. Also doubling memory width not only needs twice the memory, but you need twice the speed for everything else, to be able to handle twice the bandwidth in the same time. So larger, more expensive FPGA.. I think you get the drift..
For 100MHz/500MS/s, under 1000USD, you have Siglent SDS 1104X-E (min 500MS/s, can be unlocked to 200MHz), and you have Rigol MSO5000 (up to 8GS/s, min 2GS/s (that one can be unlocked for full capabilities )). They both have many measurements, decoding, lots of memory (Siglent has 10s of MPoints, Rigol has 100s of Mpoints.). Rigol has all that at that price only if unlocked(hacked). Siglent somes with most of the stuff legally unlocked.
Rigol is still very new and still has teething problems, but is more powerful platform. Siglent is OTOH more mature and stable and works quite well nowadays, but smaller memory, smaller screen, slower sampling rate. It is also cheaper than Rigol, so you can get a small standalone AWG for the price difference. That you can use for quite nice FRA app in Siglent scope.
There is also nice Micsig tablet handheld scope (nonisolated) that is very nice if you need portable (it will drop to 250MS/s with 4ch, but it does seem to handle it well).
Siglent and MicSig go all the way down to 500uV/div, with very respectable noise figures.
Rigol MSO5000 doesn't go lower than 4-5mV/div. So not so much for small signals.
That concludes sub 1000 USD offers that are good price/performance. There are other scopes but they violate your request for minimum 500 MS/s with all channels enabled.
What you would like doesn't exist, in that price range. Sorry.