One organisation's ideal format will be another organization's horror story.
In other words, what is an acceptable "preservation format" will differ by institution. If your institution is willing to commit to maintaining interaction software for multi-page TIFFs and/or ensuring you can move content in multi-page TIFFs to files with a newer format as needed in the future, then it could well be an appropriate and acceptable preservation format for you.
I would recommend considering those two factors (ability to support interaction tools in the future, and ability to effectively migrate content to new files with a different format as needed), and also reccomend considering what your options are for immediately normalizing the content into new files. An option that comes to mind is moving all the content in the TIFFs to multi-page PDF files that can be interacted with using Acrobat Reader. However that may cause a loss of content (depending on how it is done) and therefore may not be an appropriate approach for you. Furthermore you may not wish to support content in PDF formats and not want to commit to supporting a PDF interaction tool.