HerrJeh

Well, Apple is using good stuff on their machines, no doubt. Let this be 30% of the cost, 30% is the proprietary software platform, and the rest is „windfall“. But IMHO there is not much of an alternative (still running a PC beside my MacBook Pro, but disliking to push the power button more and more). Nobody to blame, because anybody can buy some shares to participate on the receiving side 🙂

I would definitely go with 32 gigs of RAM while you can. You have no options have purchase. Unfortunately it is stupid expensive for that RAM. Gone are the good old days when Apple computers had a balance of form and function. 
Publisher starts to bog up and slow down with larger the files. I had done a test in Publisher for another thread. I made a 400 page Publisher document and Indesign document as well as a 1000 page Publisher and Indesign document. It was nothing but text, no weird font, just the standard system font that was used by default in these apps. Indesign was no issue with either of those sizes. Publisher on the other hand began to choke up a bit on the 400 page document (still workable but a bit laggy on text input) and in my opinion was unusable at 1000 page (massive input lag). This was just text, no images, no vectors, just 100% K text.  My test was done on a 5K iMac - 4ghz i7 - 32 gigs of RAM - Radeon R9 GPU with 4098 of RAM. 

If you have to handle larger projects, more RAM is essential. It is especially helpful if the fotos in your picture books are still editable, containing layers, masks and all the rest. Since RAM is not upgradable with a MacBook, I would go for the 32GB. Affinity makes use of the GPU as well (surprisingly several other picture editors and design programs don’t), so if you configure things a good GPU can help as well. It is easier to source out disk space to a fast  external SSD, although the speed of the internal SSDS of the MacBooks is phenomenal.
My config: MacBook Pro 15“ (2019), i7/2.6GHz, GPU Vega 20, 2TB, 32GB, external BenQ SW320-Photo editing monitor. Works fluently even with large RAW projects, like pano, HDR and focus stack. With Publisher I do not have such large projects, but no issue so far.

Well, having always enough available RAM memory is essientiell for all kind of software nowadays, so APub is generally no exception here. - Though on the other side and always the bad here, the MacBook Pro RAM can't be later user updated, as in former times and Apple itself always charges a premium price for a lousy 16 GB more of RAM. So one always have to consider this (maybe) long-term benefit versus price aspect.
Personally I don't work with APub, which IMO in it's current state isn't very efficient (bug free) at all and so actually not that well suited for big mass image handlings. - However, a general rule of thumb is usually always, the more RAM the better, especially also when using a bunch of software in parallel.