Death's head on a mop-stick - using language in historical novels

As I take my characters down the Shipway (Lympne Hill) to the Sandtun (West Hythe), I reflect on the use of language in my novels.
Having just written a book set in the 1920s and another in the 1940s, I became well used to my female characters saying: ‘Gosh’ and ‘Golly’ and ‘Crumbs’ and there was even an ‘Old Bean’! They called their parents ‘Ma’ and ‘Pa’ or ‘Mother’ and ‘Father’ depending on their upbringing.
But my favourite era so far was Medieval times and when the great storm was battering Romney, they were hardly going to say ‘Gosh’ or ‘Cripes, jolly bad luck old chap’. They said ‘God’s teeth’, ‘Christ’s bones’ or ‘By the blood of Christ’. These were pretty strong and quite appropriate for if your boat or home were being smashed up by the storm.
Now I step back to Saxon times and although I write in modern day English (I wouldn’t sell many books otherwise), I want to know how my characters are going to curse if they find themselves in trouble. Well… it turns out the religious swear words are not going to happen. The best I can find is … ‘Death’s head on a mop-stick’. Hmmm, there’s only so many times I can use that in one book! I’m going to have to give up on that search.
I move onto terms of endearment and find ‘Dearling’ and ‘Sweeting’, which I like. That reminds me that I tried to use ‘Honey’ in my medieval book. It was correct for the era, but sadly sounds too American so had to go…
What else? What will the children call their parents? ‘Modor’ and ‘Faeder’ – I like that! I also have a grandmother and she is called ‘Elder-Modor’.
Now I have to consider how far will I take this, without it becoming irritating for the reader? I decide to use ‘I thank you’ and I like a ‘Mayhap’ and a ‘Perchance’. Oh and I use ‘Aye’ and ‘Nay’.
If there are any rules about how far to take this in a historical novel, then I know nothing of them. I go with what feels right for me and my characters and I hope my readers agree. I have to be careful not to slip these words into a descriptive passage, as these parts are my words written in my 21st century style.
I do frequently check words to find out when they were first used. As I write this I have just searched twine, to see if my character can tie a goat up with it, or should it be rope? But words do slip through - a character was recently well ahead of her time when she used meters as a measurement in Victorian times! I do get it wrong and that’s when I rely on my readers to let me know. The kindle edition or the next print run of books can easily be changed.
​As I pass my halfway stage in the novel, I still need to use ‘Death’s head on a mop-stick’ and I’ve got ‘milk-soft’ on my list, which I think is rather nice! 

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