What I'm trying to do is to join few arrays of dates.
For start, I have start date and amount of months I want to generate.
Which I did by using =ARRAYFORMULA(DATE(YEAR(A2), MONTH(A2) + SEQUENCE(C2, 1, 0, 1), 1)) formula, where A2 is start date and C2 is amount of months to generate.
Formula I cannot come up to is to do same for all existing rows and join all generated arrays of dates in a single column.
Also, I'd like to copy some columns next to every date to identify from which row it is. I have done this by formula =TRANSPOSE(SPLIT(JOIN(",", ARRAYFORMULA(REPT(SPLIT(D2,",")&",", C2))), ",")) which I found in google.
Also, here is what I have reached so far and you can test your versions too.