Any mood or tense might follow que here:

De là à dire qu’ils ont, ont eu, avaient, avaient eu, eurent, eurent eu, auront, auront eu, aient, aient eu, eussent, eussent eu, auraient, auraient eu aussi pour habitude de lire…

The one to use depends on what you actually had in mind.

Stephane’s comment summarize very well the rules:

As to which form should be chosen, in a few words, if the thought statement is viewed as a thesis you’d use indicative, if it’s viewed as a hypothesis you’d use subjunctive, and if it’s viewed as an eventuality, you’d use conditional.

“aient” is subjective and “auraient” is at the conditional. You use them depending of your sentence.

In your particular sentence I think I will use the indicative “ont”

Verbes d’opinion +subjonctif ou indicatif – cours