I’m not enjoying the books I am reading

Love for Lydia H E Bates

The End of Mr Why Scarlett Thomas

The Historian by Elizabeth Kostova

The Cazalets by Elizabeth Jane Howard
The Forsyte Saga

Adventure/magic theme - I wonder if you'd like Guy Gavriel Kay's books? They tick a lot of your boxes but they're fantasy - what you might call historical fantasy rather than faeries/trolls/elves etc.

Someone else recommended Possession which is SUCH a good book, thoroughly agree. Also agree with Isabel Allende's stuff.

I also love the Dandy Gilver mystery/lady detective books (1910s/20s) - a bit of a wildcard given your criteria but so enjoyable and clever, and excellent period detail and atmosphere.

Zen Cho - Sorceror to the Crown and the sequel, The True Queen. Regency era magic, first book has a strong analogue of Becky Sharp from Vanity Fair (and if you haven't read Vanity Fair that should be on your list too).

Mary Robinette Kowal's Glamourist series is Jane Austen with magic, though I haven't actually read them all, they aren't bad but didn't click for me.

Someone else has mentioned Naomi Novik's Temeraire series and I second that, and also the Patrick O'Brian books she riffed off of.

A Memory Called Empire by Arkady Martine - this is technically scifi but it's set almost all in one place in a very mannered, restrictive society that activated all my historical fiction receptors. I really loved this book so I'm recommending it anyway because we seem to like a lot of the same things.

The Golem and the Djinni by Helene Wecker - magical characters, romance, turn of the century New York. Lovely sequel too.

The Thinking Woman's Guide to Real Magic by Emily Croy Barker - magic, romance

Birdcage Walk Helen Dunmore

Alan Garner The Weirdstone of Brisingamen

Atonement by Ian McEwan?

the Don Camillo books

It's just outside your time period, and it's not exactly rural, and it's not romance or magic or ships or really adventure or crime, and it's neither epistolary nor anything to do with books

but you love Emma and Bath Tangle and The Documents in the Case so I'm suggesting it anyway. And it's not thriller or literary fiction and there's no smut

The Brother Cadfael novels too - again, totally outside your time period but there's crime and a bit of romance and possibly some adventure. I would love to have got Dorothy Sayers and Edith Pargeter discussing life and religion.

Sorry - Lady Audley’s secret not fan!

Lady Audley’s fan for a bit of Victorian melodrama.

The Plot by Jean Hanff Korelitz for a book within a book kind of thing

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The French Lieutenants's Woman John Fowles

Woman in white by Wilkie Collins? Time, a bit rural from memory and a mystery

Have you read Possession by AS Byatt, OP? It hits the book-within-a-book brief, is partly epistolary in structure and one strand of the story is set in the Victorian era.

The Master & Commander series by Patrick O'Brien. All set on ships, around the early 1800s. I reckon these are in the category of adult-Famous-Five style adventures. My go-to comfort reads.

Have you come across https://persephonebooks.co.uk/pages/book-list ? Most of the books on the list will meet at least one of your criteria. My favourites include High Wages by Dorothy Whipple and The Making of a Marchioness - Frances Hodgson Burnett.

If you like Elizabeth Gaskell how about George Elliot? "Middlemarch" is superb.

I second these books. Fits your brief well.

Seven Sisters as PP suggested

Yes to House of The Spirits by Isabelle Allende. Also One Hundred Years of Solitude by Gabriel Garcia Marquez.

Everything by Kate Atkinson.

The first book that comes to my mind from you post OP is Kidnapped by Robert Louis Stevenson. Set a little earlier than your dates (18th Century), but written in late 1800s. Set on a ship and rural setting. Adventure.

Also thought of The Familiars by Stacey Hall and The Dark is Rising by Susan Cooper.

Definitely Piranesi, it's fab. And yes to the Snow Child mentioned by a pp above.

There is also a similar style of writing to many of your likes (the ones i recognise at least), quite literary rather than holiday read. Little by Edward Carey is written in a similar style and is a fictionalised biography of madame tussaud.

I did also wonder about Crimson Petal and the White. It's great, but the main character is a prostitute so may not be your thing. However, there are no sex scenes in a smutty sense, but sex an is frequently and quite crudely mentioned

For stories within stories, you could check out Kate Mosse. These also have the period feel.

For books with a period feel, Sarah Waters. I also loved An instance of the finger post by Ian Pears and Quincunx by Charles Palliser.

The Miniaturist Jesse Burton etc

Pease try the books Laura Shepherd-Robinson has so artfully written.
I do enjoy Patricia Highsmith's work too.

Finger smith by Sarah Waters
The Crimson Petal and the White
Night Circus has quite Norrell and Strange vibes…also have you read Piranesi? Although I’m not sure I enjoyed it as much…
Is Anne of Green Gables too obvious?