I tried the solutions suggested on this Thread and some worked, but others did not, possibly because of the frequent Kindle version updating.
So, I tried out EPubor Ultimate and liked it a lot. It solved all of the problems and the perpetual license, not a subscription, is well worth the reasonable cost.
On occasion when I had any problems, the support reply was amazingly rapid and the suggestions solved the issues right away. Hence I highly recommend this product.
In case you are wondering, I have no connections to this company, nor do they offer any rewards for positive reviews. This is simply my opinion based on using EPubor for over a year or more.