I get lots of red squiggles. I can cure it for myself, but I think more of us would benefit if the spell-check dictionary was expanded in LO as shipped to users so that it included more genitives (possessives) and more words that are uncommon. Uncommon words are often left out because they're identical to errors in the work written by people who didn't have advanced vocabularies. A solution to that is to offer a rare-words dictionary that could be loaded with a single click, with an option for each user to set it among defaults in Tools > Options. Specialized dictionaries (e.g., law or medicine) could be loaded the same way. For genitives, include both the straight apostrophe and the curved apostrophe. For this report, profile-resetting and OpenGL are not applicable. Possibly related: Bug 40277, bug 87605, and bug 121284.
Nick, it sounds to me like a proposal for an extension. Perhaps it already exists? Have you checked this?
I didn't see an extension when I searched all categories for genitive, possessive, apostrophe, uncommon, rare, or law and medicine got only a Finnish dictionary and grammar checker. But I'm not clear if an extension could be designed to interact with another to produce a result not listed in either one, or to interact with a default or default-loadable dictionary. So, the preloaded English dictionaries would need modification to handle the straight/curved apostrophe and, if that's not done by adding entries (like shoe, shoe's, shoes', and curved equivalents) the part of LO or Writer that applies dictionaries might need a patch to interpret a dictionary for this functionality (like shoe[apostrophe-code]s and shoes[apostrophe-code] in the dictionary, perhaps where embedded are some Unicoode 4-byte characters not used in a natural language). Making an easier way for loading dictionaries and dictionary features would be good for nongeeks but I suspect will be objected to as redundant of the existing method for loading all kinds of extensions. Nongeeks won't think of French as an extension, so a button in the F7 Spelling dialog would increase LO's usefulness to nongeeks.
Maybe get in contact with https://proofingtoolgui.org/