Don't know if it will be useful for anyone but I use a custom keyboard layout for the Latin alphabet that has all the diacritics and unusual letters I could want, covering the classical and modern European languages and Indic romanization. It's a normal US keyboard layout until you hold down the right alt, then the white keys produce the indicated character and the grey keys apply the diacritic to the next character you type. If there's no precomposed character in unicode then you can type a character then the diacritic key then space to get the unicode combining character applied to it.
File opens in Microsoft Keyboard Layout Creator, you use that to create an installer. You can also customize it yourself in that tool.

https://files.catbox.moe/myd1dw.klc