'Franchement' does seem a bit like 'really' or 'honestly' — or as you say, 'FFS'. You'd think it would be more similar to 'frankly' as the words have the same origins, but I guess they've just diverged in meaning. Like, you would usually only use 'frankly' if you're going to say something negative, but that's not necesssarily the case with 'franchement'.

French spelling is pretty weird as it's not phonetic like Spanish or Italian, but once you get the hang of it, it's more consistent than English spelling.

I'm actually from New Zealand so everything is weird and new to me lol, but I'm really enjoying living overseas