I produce a quarterly magazine using Serif PagePlus X6. I also use it for a two sided weekly news-sheet and have used it for large (64 page plus) booklets. The export to pdf is very useful and I wouldn't move to anything else for the type of job you want. Any 'upgrade' is likely to more towards a commercial piece of software which could be quite expensive.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been