I have a two page list of proper nouns & words with diacritics that I would like imported to my English keyboard.

Of course, I could enter them manually via cut & paste, but I’m seeking an easier way. So far, no joy.

Is this even doable or have I merely done an inadequate search?

I'm not understanding what it is you want to do: "import to my English keyboard".

Just the diacritics? Not the nouns and words? You want a key on your keyboard to represent the entire word or just the diacritic?

You can set up iOS so that when you type a specific key combination it suggests a word or phrase instead.

What I hope to do Ira, for example, is when I type angstrom, I want get "ångström".

And I can get that if I enter each foreign word individually in Keyboard/Text Replacement. It is that tedium of entering each foreign word in Text Replacement that I hope to avoid.

OTOH, I could do 10 entries a day and be done in <2 weeks. What, no pestering and fussing?

If you want to enter words at your keyboard using "unicode" characters, I suggest you start by enabling your keyboard viewer. Apple LOVES to move this one around unfortunately, it's had several homes and appearances over the years.

select apple, system preferences, keyboard, input sources
check "Show input menu in menubar"

You should now have a new boxy icon up by your clock, click on that and select "Show Keyboard Viewer". In prior versions, it was under International, and you had to enable the menu AND enable the keyboard viewer to be added to it.

This will give you a floating keyboard that will show you in real time what keys are being pressed. If you hold meta keys like Option, you will see most keys change. What you see is what you will get when you hit that key with the option key down. Shift and/or control will also vary the selection somewhat. I use these most often when I need a euro or a tilde N etc.

Some keys will be orange while holding Option, those are your diacritics. Press the key for one and release the option key. Some keys on the standard layout will now have diacritics added to them and you can use them by pressing the corresponding key.

You can also click on keys in the floating keyboard using the mouse if you wish to. If you have more than one keyboard attached, the last keyboard to be used will be displayed, so tap shift or something on a keyboard to switch to it.

Once entered, most apps on the mac are unicode-aware, and you can cut and paste them normally.

Different diacritics will be available depending on which language of keyboard you have selected, and not all of them are available on the US Keyboard layout. You may need to add that keyboard to the input sources (where you checked the box above) and select it from the menubar to access character options used in that language.

As always V1, I learn much at your knee. Often that is more satisfying than resolving my original problem.

In this case, since I already have the list of words (with diacritics) in a text document on my iPhone, I've opted to merely cut & paste them from that to the Text Replacement.

Still, I wish Text Replacement had an import function. Maybe when I'm King...

Yes, informative. But is there an iOS equivalent, which I think is what the original poster was looking for?