You can have up to 9 external lookups.
Can any or your external lookups be changed to internal lookup tables? If they rarely change this could be a good idea.
Alternatively can any of them be combined to make one lookup where previously two or more were needed?
Extreme cases may indicate the benefit of using several stages for a development with a new intermediate 'database' of some form being created by exports at each stage. Such a concept may be easier to develop and maintain than a single complex model.