Does anyone have a straightforward, step-by-step guide on how to publish all the way from Dorico to Apple Books? It feels like it shouldn’t be this hard! 
I have two separate projects that I want to put together into one final file.
My plan is to use Apple Books and maybe Google books because I want to make several updates over the coming years and make it easy accessible worldwide, but I am open to any other suggestions on where to publish. Thanks!
I have experience with this, just not really the time at the moment to go into deep detail.
The TL;DR is that it is not straightforward. You either need InDesign (or another DTP program) to get things to export right(ish) to EPUB, then another program to open the EPUB archive if the Apple publishing platform protests (very often). Just know that, if you use your PDFs as they are, they will be converted to PNG (or other raster format) and resolution will be very bad or, if good enough, the image size will be rejected.
It’s a royal PITA, honestly.
A self-publishing platform where you place your PDFs could be a friendlier starting point.
I agree with @MicheleGalvagno . I have not used Apple Books in years, so it might be much better than it was back then; it was almost impossible to do something productive with non-text items.
EPUB isn’t much better. It’s - what I understand - basically HTML, and images are handled poorly.
I’d go for issuu.com, which isn’t free but is a major player. They also have ‘digital sales’.
Otherwise, if you just want to make it public, how about https://archive.org/ They also have a “flip book” reader.
or ISMLP
Thanks so much for help! I will check out the other options! 
Thanks so much for help! Seems like its best to try something else yes!
It’s fairly easy to join two or more PDF files together into a single file, especially on a Mac using Preview. You can even enter blank pages between files if you wish. I do this frequently. With a little manual work, one can add sequential page numbers, create a TOC in Pages that is exported to a PDF and end up with a nice looking large document. Note that the page numbers is a manual process of adding a markup to each page.
Perhaps not with a Mac utility, but PDF editors can often add footnotes containing page numbers and even recalculate them if the page order is changed. That would save some effort over a manual process.
Thanks for help! 
this is free and you can add pagenumbers, headers and footers.
To add to @Maarten_Kruijswijk,
This was just updated to v1.7.0 in case you missed it!