Soulver is amazing, and totally changes the way you use a calculator.
Alternatively, I use a lot for a long while Tydlig. Sadly it has been stagnant for a while (I'd like to be able to save "sheets", for instance). But its interaction mode (akin to a spreadsheet, in a sense) is very refreshing.
+1 for tydlig, among the first apps I install on every new iPhone
Can I have one as a desk accessory like the original Macintosh (or some DOS TSRs), ie. a floating window while we're doing wish lists?
PCalc works as a notification widget - so it looks like you can have it floating on the new iPadOS homescreen - or just pull down and swipe to access it everywhere else