I feel like I am beating a proverbial dead horse by adding my voice to this thread, but it nonetheless seems worthwhile to add my perspective to the other voices who have chimed in here.
My background is that I have nearly three decades of experience dealing with CJK character sets, CJK encodings, CJK text layout, and CJK font development, with a very strong focus on the Unicode and Japanese aspects of those fields. I worked at Adobe for 28+ years.
I should also state that I bought a license for Affinity Publisher late last year, but I generally do not use it mainly due to the lack of this very fundamental feature. In addition to supporting the appropriate OpenType features, in particular vert (Vertical Alternates), vertical text frames need to be added. There was also mention of supporting ruby, which is also referred to as furigana. While this feature is certainly used in horizontal layout, with the 50% glyphs floating above the characters they annotate, there are special considerations for vertical layout in terms of their placement relative to the characters they annotate.
My daily-use layout app is Adobe InDesign, and have mine set up to expose full Japanese (and CJK) functionality. I am fortunate to have a lifetime Adobe Creative Cloud subscription, but decided to give Affinity Publisher a fair shake, which I did.
While my entire book, CJKV Information Processing Second Edition, would certainly be helpful, Chapter 7, Typography, in particular would be the most helpful for the Affinity Publisher developers. Please find attached the PDF for Chapter 7 for your reading pleasure. While the book was published almost 12 years ago, its content is still completely relevant today.
I suggest considering a Baby Steps™ approach, such as to start by adding support for 1) vertical text frames; 2) the OpenType 'vert' feature; 3) respecting the metrics in the OpenType vmtx (Vertical Metrics Table) table; and 4) the 90-degree clockwise rotation of Western text in vertical text layout. Also, be sure to study UAX #50, Unicode Vertical Text Layout.
Adobe InDesign, with its full Japanese functionality exposed, is the most sophisticated Japanese text layout app on the planet, meaning that it represents an extremely high bar. Still, it is not without its flaws. When I typeset my book over 12 years ago, using Adobe InDesign CS3-J, I faced many issues. While I was able to resolve or work around all of the issues, I felt compelled to write a two-page White Paper that detailed the issues I faced. Interestingly, although it is 12 years later, many of the issues have not yet been resolved. Of course, I am using Adobe InDesign 2020, and will have access to the 2021 version in a little over a month from now.