I loved this manga 幸せな家庭が壊れるとき～理性がブッ飛ぶまでイカされ続けた母と子の末路～ https://www.dlsite.com/maniax/work/=/product_id/RJ317121.html and I couldn't find anything even closely related, except the "Nightmare Express" series.
I wonder if you know anything dealing with this theme?
i cant read japanese so the one you liked the most i wont be able to read but here is one i like with simular story and its translated to english
by the name of [Sannyuutei Shinta] Okaa-san wa Shakkin Dorei desu yo My Mother Is A Debt Slave [English]
google it you would easily find it and if it is what you needed then i have more unless you need the gang to actually do the son as well whiche is some thing rar and hard to find
still there is a hentai where the mom does her son with a strap on
title :
Furyou ni Hamerarete Jusei Suru Kyonyuu Okaa-san
there is a visual novel :Ecchi na Okaa-san to Otoko no Ko to Sono Kareshi https://vndb.org/v19940
but with no gang just the mother bf doing both she and her son
there are some ntr anthology with the same setup but i cant find it know maybe later
