Why "dobrA rzecz" rather than "dobrY rzecz"? There is nothing in this sentence to show feminity...
Rzecz is a feminine noun
Hmmm... I feel that I'd need some more context to interpret "thing" as "sprawa", which generally is rather "matter".
Can you use this sentence with "thing" being a situation like in English, "It is a good thing, that you got the job" or "It is a good thing that you went on your holiday abroad before they closed the borders"?
Yes, that's almost certainly what is meant, rather than a literal "thing" (an object).