i think it is relatively easy to do it with gpos.
You have to download the firedox esr gpos and put them into the folder windows/Policydefinitons and there is then a policy for the proxyconfiguration where you put in the information you entered manually.
To make the gpo work for the domain, you have to install gpmc on a admin windows pc and then you can configure that.
I can provide more details if needed.
However, if you want to use linbo later anyway, it might be the better option to setup linbo and put the settings in the default user profile of the linbo image. and then you should be set too… it is a little work to get the windows image working… but it might be worth it…
the manual domain join for all the pcs is also work… You still have to fill the devices.csv. so if u had a working linbo image you would save the work of the manual domain join for every pc, right?