bob_janes

Just as a side note : you can also do the reverse, installing Ubuntu as you main OS and use VirtualBox to run Win10 inside a VM ...

I don't agree with desktop horrible performance. I already use an s912 board as a small dev computer and it's a bit slow because of mali performance with a browser, but otherwise it works well.
rk3399 is better on all point. I just bought a khadas edge-v and I plan to use it instead my s912 box. thanks to JMCC script, I expect enough performance for a smooth desktop usage:
 

I haven't this board, but here is my opinion about your questions:
0) 2G RAM may be enough since zram can increase it and it's activated by default in recent images.
1) I don't see real hardware differences between your current config and the new one, but I can't give you any guarantee it'll work.
2) It mainly depends on your use case and the loads you expect on the board. But it'll be much more powerful than your existing hardwares. I'll recommend the aluminium heatsink to avoid too much throttling.
3) A quick search on google gave me a like to thingiverse: https://www.thingiverse.com/thing:3313324, but you have to print it. The main problem is the sata hat, and the way to integrate your external drive.

NanoPi M4 and Rockpro64 have sata card extensions. You can have a look at them to see which one fit at best with your needs.

I would definitively not put all these stuff on one board... If something messes.. your entire network will be in trouble including all the data on the nas.
 
check the PSU, if it's only that.. the board can still serve as a NAS, and thanks to the new patches for its sata implementation.. it shouldn't be a bad one..
 
rk3399 is powerful for a lot of your other tasks.. but still wip.. and things might change here.. means the risk is higher that things break as well. for the blog/webserver stuff.. As long as you don't expect much visitors there.. a cheap H3 might do it.. and for pihole no idea.. probably one with GbE, not my field..

With this budget, any well supported board with rk3399 and a good sata2usb3 converter will make the job.
The NanoPi M4 with its big heatsink can be nice for less than 72$ without shipping fees. For similar price you've got also the rockpro64.
The Odroid N2 (or Khadas Vim3 launching soon) could be nice also, but amlogic S922x is a recent soc and current boards are not officially supported event if there are good progress.
 
If you absolutely want a sata port, an odroid hc1 is cheap and interesting but there is no hdmi output. For both sata and hdmi, you can look at the Odroid H2 (based on intel J4105) but it's out of stock and you have to add ddr4 ram. The main advantage of this last one, is that you are on an x86 soc, so any "standard" linux distribution may work quite out of the box.