Question... will electrs download/build its own DB now? I guess yes, but will that be about 20-30% of the bitcoin DB?
Yes, it builds it's own database... my "electrs" dbDir is currently ~56gigs. Note that this in addition to the Bitcoin Core DB (which is currently ~324gigs)... you still need the Bitcoin Core DBs as well! 
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Also, how about WLS moving it to another PC? is this possible?
Apparently you can... the WSL command (included when you setup WSL within Windows) has an --export command which will export a specified distro to a "tar" file and an --import argument that will allow you to then import that distro to another PC...Quote
Arguments to manage Windows Subsystem for Linux:
--export
Exports the distribution to a tar file.
The filename can be - for standard output.
--import
Imports the specified tar file as a new distribution.
The filename can be - for standard input.
--list, -l [Options]
Lists distributions.
Options:
--all
List all distributions, including distributions that are currently
being installed or uninstalled.
--running
List only distributions that are currently running.
--export
Exports the distribution to a tar file.
The filename can be - for standard output.
--import
Imports the specified tar file as a new distribution.
The filename can be - for standard input.
--list, -l [Options]
Lists distributions.
Options:
--all
List all distributions, including distributions that are currently
being installed or uninstalled.
--running
List only distributions that are currently running.
NOTE: I have not used this, so I've not experience with actually using it. I'd suggest trying Google for more info.