Kindle finally supports ePub but will kill MOBI

You will no longer have to use a 3rd party website to convert your ePub to something Kindle can read. This change has happened already, however soon, MOBI, which is a format many sites handily converted ePubs into for sideloading to a Kindle, will be disabled for new uploads. Your previously uploaded MOBI files will still work.

I have never had good luck with books converted by Amazon looking right, however, external conversion sites were wonderful.

Ars Technica:

Kindles still don't natively support sideloading ePub files—the Send to Kindle service converts the documents to AZW3 files, something users could already do themselves using a variety of free tools. But official support from Amazon removes a step from the process and will help users avoid shady ad-riddled third-party conversion sites. We've tested it with an 11th-generation Kindle Paperwhite running the latest version of the Kindle software, and the conversion process seemed to go off without a hitch.

If using the Send to Kindle email address is still too onerous, Amazon's support document says it will add the same ePub conversion support to its Send to Kindle apps at some point in late 2022. Amazon also says that it will discontinue Send to Kindle support for MOBI files in late 2022, since those files "won't support the newest Kindle features for documents." MOBI files already on your device will remain there and will continue to work as they currently do.