The modern historical novel began in Romanticism: escapism to other times rather than exotic places. If it first blossomed with Sir Walter Scott’s Waverly, it steadied and lost its heavy poetical load by 1840 with The Last Days of Pompeii.
Decent histfi requires research. Lots of research. More than you can do by reading websites.
Besides “general purpose” multi-era fast references, expect to read at least 40-50 new books for a new period. If you don’t like reading history, why would you want to write histfi? Set your wonderful story of adventure and romance in a non-magical imagined world (the “hard fantasy” or soft scifi subgenre of science fiction) where history is what you want it to be. Try it: you’ll love the freedom to make this near-parallel world exactly what you need!
Movies can zoom past anachronisms. Movies are made for a mass audience with zero critical threshold on historicity. Quite frankly, many of them are hard fantasy, set in parallel worlds with the same city names, like–
• Ancient Roman and Renaissance armor and 19th century draft horses in The 13th Warrior.
• Gladiator‘s Marcus Aurelius that does not adore his son and live for the idea of him being emperor.
• The Xerxes costumes in 300. Or Spartans with short hair and mustaches — this is so wrong.
The historical novel audience is smaller and pickier. Readers wanting medievalesque fairytales may pick up any medieval romance, but medieval-re-enactor ladies reject most of those. Sitting on the sward during sword-fights, they have nothing better to do after they run out of Kingdom gossip than tear authors to shreds. Something good will get shared, but not loaned, and others will buy it the next week or so. So devastating accuracy pays off, literally, in higher sales.
Cultures often have iconic periods so frequently used it seems “almost home”: Americans, Westerns; Japanese, the Tokugawa period; French, the Thirty Years War; Italy, the Renaissance; England, the Plantagenet period. It’s easy to get sloppy about these because you think you know it without research. When you write histfi, you have to get details scriptwriters leave to the designers or needn’t bother with at all. You have to know how clothes constrict, how streets smell, what is good or fashionable to the natives, and why, because you are taking the readers inside their heads, not just watching them walk by as a fly on the wall. When you use these minor bits, you so convince readers of your authority that they quit questioning and just settle in for the ride.
For example, American West: cowboys step up into the saddle from the near side of the horse. I mean, mounting on the left is so “universal” that we can call that “the near side,” right? But Plains tribes mount from the opposite side. (So, for that matter, did the Ancient World.)
You have to decide, and sooner is better than later, if you want to do research or world-creation. Some people will tell you the research is easier, once you realize how much work rational geography, sociology, etc. are. You need to read a lot of real-world to learn how invented cultures and ecology might work.
Besides, doing historical research gives you all kinds of sand for pearls that makes your histfi almost write itself.
But, if you are committed to your plot and it just won’t work in the period as you discover what it really was like, consider that soft scifi alternative. Period laws are something people often don’t get into until late, while their plot depends on some point on the law, say, of inheritance, which they find out didn’t work the way they thought back then. Doing an invented world lets you keep the plot intact, plus you can now gleefully create all kinds of other problems in your new creation. The research already done provides a solid basis for building a plausible divergence.
A writer really can’t just hand-wave it away as “I’m doing alternative history.” It won’t sell as that if the only alternativeness is that woman can inherit noble titles but are still Victorian damsels in distress, or basically the author is just inflicting 21st century or imagined attitudes on the period. That’s an excuse they’re making, not a genre choice. Althist is a genre that requires all the historical research, then the world-building for an important divergence, and the plot has to pivot on the divergence, not just a side issue.