Should we respect the origin and say “Several tsunami hit the coast”
No. In English the rules of English grammar apply. Of course, we follow what people do, not what the “rules” say people should do, but the pattern is that, over time, foreign borrowings will take on the English plural.
We didn’t add an s to coterie (Several coterie combined to...?) being more familiar with French when it entered the language.
Huh? Coteries is the usual plural in English.
Regarding dim sum, a pattern in English is for food to be considered in the singular, “we’re having stew,” “we’re eating cake,” “we bought pizza.” The same thing is going on with dum sum. And I have heard dim sums when referring to a number of the savories.