I’ve been having some issues with it for the past week or so, likely as the result of the upgrade to Mastodon 4.0 and the increases in traffic, but the best/only method for syndicating from my site into a separate Mastodon instance that also includes alt-text for photos is Brid.gy (see: https://brid.gy/about#picture). If set up in conjunction with Webmention plugin (and optionally the Semantic Linkbacks plugin), Brid.gy will also feed back replies/likes/boosts from Mastodon back to your website as comments. If you do use Brid.gy to syndicate/publish into Mastodon, you’re going to want some basic proper microformats support for your theme. I’m happy to help describe this in a crash course if you don’t have the expertise on microformats, though I know you’ve got the theming background to pull it off. (See also: https://indieweb.org/WordPress/Themes#Microformats)

Two other options here are:
* to use https://fed.brid.gy/ which will go a long way towards making your WordPress site look like an ActivityPub/Fediverse instance, or
* use Matthias Pfefferle’s ActivityPub, Webfinger, and NodeInfo plugins which will give you a lot of the same functionality (this is what I’ve opted for making my whole site followable as @chrisaldrich). This option also automatically adds alt-text to photos if they have them in your own website.

I haven’t compared the two options in over 18 months, but I think Pfefferle’s may give stronger/broader support presently. The thing these don’t give you is a reader interface for subscribing/reading/direct interactions, but that can be done reasonably with a feed reader, or even better with a social reader. I’ve outlined some of this functionality in the past: https://boffosocko.com/2021/04/26/a-twitter-of-our-own-at-oerxdomains-2021-conference/