“Actually it would apply to having been hosted for Shabbos in a 3 room illegal basement apartment in Boro Park as well.”
no it wouldn’t.
“This is the Yekke side of my parentage speaking.”
there arent many yekkes in Boro Park (and probably none in illegal basement apartments).
Manners are determined by convention. In a society were after a meal the custom is to redo your hosts home then that is expected. If the custom is to mail a thank you card then that is expected. If the custom is to say thank you on the way out then that is expected.
In Boro Park the custom is to say thank you on the way out, not to redo the home nor to mail a thank you card. In Connecticut it very well may be different,.