900 pictures is quite a lot. What size is each picture, both in pixels as well as in kilobytes?
If I assume each picture to be about 1 inch x 1 inch or 25.4 mm x 25.4 mm = 645,16 mm² then the total area of all 900 pictures would be 580,633 mm². That’s far below the limit of Papyrus at 9,000 mm x 9,000 mm = 81 million mm² and so you shouldn’t run into limitations from this side.
If each picture takes about 70 kB of memory (in .jpg format) then all 900 pictures combined will make 63,000 kB or 61 MB which I would assume ebook readers can still handle.
As for creating the book, the main content seems to be a catalog of coins with some information about each coin. You will have the biggest flexibility if you create a database with Papyrus Base which stores all the images and information. Then the catalog pages can be generated as a report from this database. This report is a papyrus document itself and it may only need a title page and some front matter or whatever additional information is needed.
Since you can create several report templates for the catalog pages you would be able to play with different layouts and arrangements for the book.
For example, it might be good to try a layout where the information about the coins is displayed in a small, separate table but the pictures themselves would be under the table in a normal text line. This would be easier displayed on a small ebook reader screen. Also, the notes could be displayed under the pictures or under the information table.
All these different variations can be easily tested with separate report templates as only the template needs to be adjusted and then all the catalog pages can be generated within seconds. Then export as an ebook and test it on an actual device.