There isn't a SS2 Public Discord to invite you to.
This is actually the best place to ask for help.
There is a Patreon one called Sim Settlements Supporters, for the folks that donate through Kinggath's Patreon:
Sign up
And another for providing support to folks who would like to try their hand at building plans, and other modding.
Re building in Headquaters, you may wish to find some rooms that can be built: Living Quarters for 2 people or the Department Heads.
Here's a link to
Headquarters information. From that page, there is a link to the floor plans and rooms that can be built.