Extensive diacritics input in iOS/iPadOS MacOS has an input method known as "ABC - Extended", which allows inputting most diacritics with any Latin letters. With it, "Alt" + various keys give a diacritic waiting to be composed, and pressing a letter produces the letter with the specified diacritic. For example:

Alt + E gives acute (´), which can be used to produce é or ń

Alt + U gives diaeresis (¨), which can be used to produce ü or ÿ

Alt + C gives cedilla (¸), which can be used to produce ş or ķ

Alt + B  and Alt + V give breve (˘) and caron/háček (ˇ) respectively, which can be used to produce Romanian ă and Pinyin ǎ.

My question: is there a similar way to input various diacritics on iOS/iPadOS? All of the system keyboards seem just to offer a limited set of accented letters.

.
The best way to have full coverage of diacritics is to get a hardware keyboard to go with the iOS device.  Then you can just choose ABC Extended as your mapping.

Otherwise you have to search for a 3rd party keyboard (check the Keyman app) or use an app like Unicode Pad.