The best way to do math on my phone I know of is the HP Prime emulator.
Honestly, the main beef I have with Calculator.app is that on a screen this big, I ought to be able to see several previous calculations and scroll up if needed. I don't want an exact replica of a 1990s 4-function calculator like the default is (ok, it has more digits and the ability to paste, but besides that, adds almost nothing).
Also it does some level of symbolic evaluation: sin^-1(cos^-1(tan^-1(tan(cos(sin(9))))))== 9, which is a better result than many standalone calculators.
Also it has a library of built in unit conversations, including live updating currency conversions. You won’t see that on a TI-89!
And I just discovered it actually has a built in 2D/3D graphing ability. Now the question is it allows parametric graphing like the MacOS one…
All that said, obviously the TI-8X family hold a special place in my heart as TI-BASIC was my first language. I just don’t see a reason to use one any more day to day.
I'd like multitasking too with multiple apps visible at once so I could copy figures easily from one app to another, like the Android I tried in 2020, but obviously that's asking too much of Apple.