I find myself often winging it with “themself/themselves” and it seems to be like themselves is always colloquially correct when there are multiple preceding nouns you’re referring to…
Otherwise if there’s only one antecedent or whatever, its themself
Be gentle haha
But there’s no “correct” pronunciation of any word. Languages change organically.
AIUI, the spoken /t/ in “often” gradually disappeared over the centuries but has more recently made a comeback due to the prevalence of text communication in modern life. And perhaps also due to so many non-native English speakers, who tend to pronounce letters when they see them.