With the following context :

Pierre est né en 1938 dans une famille pauvre. Il avait trois frères et quatre soeurs. Il ___ une vie difficile. Son père est mort quand il avait 5 ans.

You can continue with the tense you were using : “Il avait trois frères et quatre soeurs”.

To avoid repetion you can write :
“Il avait trois frères et quatre soeurs et vivait une vie difficile”

The nuance here, as a french speaker, is that I would use “Il avait/vivait une vie difficile.” for someone that is still alive today and “Il a eu/vécu une vie difficile.” for someone deceased. The reason is that it is a more proper way to address the past with the imparfait when it is true in the present and use the passé composé when it is not. If the person in question is deceased, then he/she is not living in the present and therefore i would use the passé composé. Nonetheless, both are “correct” theoretically speaking.

In this context, the most natural to me (native French) is:

Pierre est né en 1938 dans une famille pauvre. Il avait trois frères et quatre soeurs. Il a eu une vie difficile. Son père est mort quand il avait 5 ans.

I think it’s because it feels to me like this text is going to “historical”. If I wrote “Il avait une vie difficile”, the narrative point of view would be different, I would expect the rest of the text to be placed at that point in time where he is experiencing a difficult life.

Interestingly,

Pierre est né en 1938 dans une famille pauvre. Il a eu trois frères et quatre soeurs. Il a eu une vie difficile.

doesn’t work as well. Probably because the fact of having brothers and sisters is a-temporal (I’m just guessing here).