Regarding birth control for non-medical (e.g. financial or other) reasons…
The Shulchan Aruch (Even Haezer 76, 6), Rambam (Ishus 15, 1), Chazon Ish, Maharam Schick, etc clearly states it is assur for a married couple to postpone the mitzva of pirya vrivya.
The Chazon Ish amongst others states postponing the fulfilling of this mitzvah is Bittul Hamitzvah even if one later fulfils the mitzvah.
In Igros Moshe Even Haezer vol 1 beggining of no. 64, Reb Moshe states that “spacing” children is completely assur.
The Gemora in Yevomos 62b says that even after fulfilling pirya vrivya (i.e. one boy and one girl) one must continue to have additional children. This is based on Koheles 11:6 vloerev al tanach yodecha. In the morning you shall plant your seed, and in the evening you shall continue to do so.
The Birkei Yosef in Even Haezer chap. 1 writes you cannot decide to discontinue having children.
And today the wife has the same obligation as the husband of pirya vrivya (Lev Avraham #99 from the Chasam Sofer), since today the husband can only have one wife, whereas previously the husband could have multiple so if one wife wasn’t having children the husband could fulfil his obligation with other wives. The Avnei Nezer says this fact was true even before Cherem Rabbeinu Gershom.