Can I extend my O-A multi-entry visa at the Thai embassy in Sweden?

How about this..

Before you leave the country (and before 18th Sept 2019) exit/re-enter the country to get a NEW year admitted stamp good until 2020.

Then before you go back to work and leave the country buy a re-entry permit for 1000baht.

That would keep the stamp 'alive' so IF you came back during that time you would be stamped in until that date again.

OR

You could just buy a brand new year-long, multi-entry Non-Immigrant Type O-A visa from the thai consulate in Sweden BEFORE you come back

And this year-long, multi-entry Non-O-A visa was issued in your country BEFORE you came here right?

What is the "enter before" or "valid until" date on the VISA itself?

Nope, you cannot get extensions outside of thailand. You get extensions at the immgration office INSIDE thailand.