Should have no issue at all getting the 60 day visit thai family extension at the immigration office where you stay for 1900baht
Won't affect the validity of the VISA in the least.. You can still bounce out and back to get new 90 day entry stamps on it until it expires.
Keep in mind different offices interpret the clause that allows 60 day extension for visiting family (clause 2.24 police order
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) in different ways..
SOME offices allow you to get that 60 day extension ONCE PER ENTRY <- meaning you can take it every time you enter on that 90 day visa
BUT
SOME offices allow you to get that 60 day extension only ONCE PER VISA <- meaning you can use it once on ANY entry (and if that's the case you should hold it and use it on the last 90 day entry you make on that visa)