Maybe you could use Lord of the Rings as a blueprint? The Harry Potter books feel episodic, so I don't think it follows the same pattern. If you're not able to introduce that sort of arc to this, you might consider publishing the series as a serial instead. With that, you'll have the freedom to tell the story sequentially without needing to make them stand alone on their own.
If you choose that route, you can divide it into arcs instead (Allies of Old Book One, Book Two, etc), and with a hundred or so pages for each one. You can do a monthly serial to keep people on the hook. I forgot which podcast I've heard it on, but the serials they produce actually make more than normal books since what should be a normal novel is divided into more. Let's say The Allies of Old is made up of five books but can be divided into twenty or so different arcs. You wouldn't need to cram side stories into larger stories, you could just spin them off into their own novellas.
I've actually thought of doing that years ago, but I became employed and had to put writing... on the shelf (ha!) for a while. If I remember correctly, that podcast was hosted by three people, and they cycle with each other to co-write stuff. Author A has his own books, but also have a number of series with B and other series with C, the same goes for the other two. Ah! Collective Inkwell is what they called themselves. Wouldn't it be fun to have that sort of thing as well with Boom, Paul and others? We would be publishing under one banner, but we all have our separate independent imprints as well. Like, you have your individual one, then you have one with Boom and one with Paul, then one with me, etc. The possibilities are endless!