We do not expect the Citadel to start building Relays in the next decade.
You should. Even in canon there were characters who thought the Asari republics could build a Relay if they were motivated to actually put the effort in.
The basic tech already existed even before canon, as evidenced by the fact that comm. relays are explicitly just very small Mass Relays without the fancy armour and Reaper software.
Given the new power generation and materials we have already introduced and the resulting economic boom new Relays are almost certain to be a thing whether we personally build them or not.
Or to look outside and see the Quarians built a mobile planet.
Why would they want a mobile planet?
That said there are very likely to be doing
something productive.
As I already pointed out they are probably making their own Arc Reactors, at least in limited numbers. I would not be at all surprised to see their fleet be in significantly better condition in a decade, especially if they get their hands on the fuel-less engines we plan to introduce.
We don't expect Nyarlathotep to show up at PI HQ to serve a restraining order on Revy for molesting physics and spacetime, either.

As Nya isn't part of either setting I would agree.
Doesn't mean I don't worry about The Mandarin turning up to screw with us if the GM thinks it would enhance the quest.
The advent of a faction which can selectively deactivate Relays in this universe distorts the setting at a level where it becomes the story, and the GM will frame entire plotlines around the universe's response to this.
One of the core foundations of the setting is not going to change just to provide us a mid-level challenge.
At what point did I indicate it would be 'a mid-level challenge'?
The GM introducing something that cripples the majority of our production is, more or less by definition, a major difficulty spike.
We put major/critical production facilities where there are heavy defences
-snip-
I mean places with Fleet bases.
Your argument is invalidated by the simple fact that all existing defences are rapidly becoming obsolete to the point of worthlessness.
The only way we can heavily defend our stuff is if we build those defences ourselves. We can do that anywhere.
Exists in a single-planet environment with tech limitations ME does not have. The conditions are too dissimilar for useful comparison.
the SA and PI have to worry about surprise attackers dropping out of FTL in weapons range of facilities.
This will be an issue regardless of how concentrated or decentralised said facilities are.
Putting everything in one place means a single loss is crippling.
You can be damn sure that during a war, and even after it, the location of production facilities by a major SA RnD/military supplier like PI will be subject to a lot of government "suggestions".
So what? We are already powerful enough to tell the government to take a hike. Even ignoring the possibility of threatening to shift our focus to supplying the other Citadel races.
Come to think of it if you are looking for the safest place to build factories then we should be trying for
Palaven, not Earth.