Hmm. That’s an interesting problem to have.

On one hand, I can’t suggest a great alternative, but man, silicone keys…I guess if they work for the author.

Stenotypists – people who have to professionally do very high-speed text entry – do use these dinky specialized keyboards that IIRC from a Japanese-language one – I think that there were multiple Japanese layouts – can only have a home row or something. I think that they use chording or something. I don’t know if that might address it, but learning one would be a huge change. Also, I have no idea what keys they can output…given that they’re highly-optimized for text entry, they might not be able to do weird symbols.

goes looking

https://en.wikipedia.org/wiki/Stenotype

A steno machine, stenotype machine, shorthand machine, stenograph or steno writer is a specialized chorded keyboard or typewriter used by stenographers for shorthand use. In order to pass the United States Registered Professional Reporter test, a trained court reporter or closed captioner must write speeds of approximately 180, 200, and 225 words per minute (wpm) at very high accuracy in the categories of literary, jury charge, and testimony, respectively.[1] Some stenographers can reach up to 375 words per minute, according to the website of the California Official Court Reporters Association (COCRA).[2]

Hmm.

Looking at the key layout there, and here:

https://stenokeyboards.com/

…it looks like English-language stenotype keyboards don’t just use a single row, but rather two or more rows. So that’s probably out.

There’s apparently a second chording layout, the “palantype” layout, but that also doesn’t do only one key per finger:

https://www.openstenoproject.org/palantype/tutorial/2016/08/21/learn-palantype.html

There are dedicated chording keyboards that do use only one key per finger, though.

https://en.wikipedia.org/wiki/Chorded_keyboard

That has some examples of one-key-per-finger keyboards, like the BAT keyboard (well, that has three for the thumb, but given that you hit those with the side of the thumb, I assume that it’d be okay with long nails):

https://www.infogrip.com/bat-keyboard.html

The problem is that (a) the BAT is discontinued and (b) you really don’t want a one handed keyboard, which is what the BAT does…it’d be better to have a two handed chorded keyboard, or you’re taking half of your fingers out of the picture.

EDIT: Here’s an open-source, two-handed chording keyboard, the Ialboard, based on the discontinued DataHand keyboard. I’m not sure that it’d work with very long nails in its current form – they might collide with the structure of the keyboard – but it’s 3d printed and I’m pretty sure that if the format doesn’t work as-is, a tweak to the 3d-printed keys would permit for arbitrarily long nails. Just need to create a space for 'em.

Gonna need some serious keyboard re-learning, though.

EDIT2: Here’s another two-handed chording keyboard with one input device per finger, the CharaChorder. It uses an analog D-pad under each finger. I think that it’d have space for long nails as-is. It does have separate arrow key and mouse control sticks, and I’m not sure if those, given the placement, would be an issue for long nails. I’d imagine that if one were determined to work around that with an external mouse device and – if you use arrow keys enough to need it; I rarely do – maybe some dedicated arrow key keyboard, though I’d think that having the arrow keys in vim-style hjkl-style layout might be preferable to the inverted-T layout that seems common. I don’t know whether the barrel connector’s positioning there would be an issue, though the positioning of that is obviously not critical to the keyboard, and I imagine that if it is an issue, with a bit of work, one could relocate it.

EDIT3: It looks like there’s another DataHand-based 3D-printed keyboard besides the above-mentioned Ialboard, the Svalboard. Same argument as with the Ialboard – I’m not sure that it’d be usable with long nails as is, but I’m pretty sure that that design could be modified with approprately-different key shapes to permit long nails to extend through a gap. You can apparently buy the thing in kit form, 3D-print it, so I figure that if someone wants to make a “longnail” variant of a few of the 3D-printed parts, that’d probably be a pretty easy keyboard to start from.

The downside is that I don’t think that – unlike the above CharaChorder – this comes with a travel carrying case, which might be important if you’re a laptop user. I guess it’d probably be possible to craft something with foam and a shell, but that’s not off-the-shelf any more.