I miss configurable thickness or have I just overlooked it?

It doesn't exist...yet. The thickness is automatic from grid size. The main reason is to not have a too long list of options that aren't very useful, thus making it harder to find and configure the more common settings.

But I have thought about adding it. Maybe condensing all grids options for thickness into a single string would make it less obtrusive.

I would not make one option for the tree grids, this is confusing.
but if you really want to merge something. merge options into one field and parse as table, However, this makes it more difficult for the user to keep the format correct.

local str = "{ key1 = 'value1', key2 = 'value2', key3 = { subkey = 'subvalue' } }"
local tableFunc = loadstring("return " .. str)
local parsedTable = tableFunc()

That just seems very hard to use for 3 numbers. I'm only talking about having all thickness settings on 1 line, not all settings. Like "25 10 4" for the 3 grids. All current settings would continue to be as they are.

that was just an idea, because you said you had too many options ;-)
what i meant was, if you now have separate options for all 3 grids, and now you start using only one new option (with 3 numbers for the grids), it's confusing. because that would be a break in the operating concept.
These are just my thoughts as a UX developer :-)
In the end, I personally don't care about the solution, because I configure it in my personal (override) mod and hide all options I did override.

Yes, I also don't like it because it would be different from the other options. But I don't really have any control over the GUI since Factorio forces the presentation, so I can only choose bad options.
So currently the least bad and most sensible has been automatic thickness to the perfect values with no need for configuration :^)

I haven't got round to the GUI changes yet, I'm still busy with the other API changes.

at present i use the 2nd grid as a ‘street’ boundary, so the thin black lines don't bother me that much.
https://prnt.sc/9ZTgFm1gBHZ8
when i have time i'll probably have to look into the code myself to see what i can do with the thickness

I'm not strongly opposed to thickness settings. Somewhat likely I'll just add it when I start playing the game again.