Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- A good practical solution for your problem is to have intimacy after Fajr Prayer so that you can have a plenty of time till Zhuhr Prayer.
2- In case you fear health problems due to washing your hair, you can perform ghusl without washing your hair and perform tayammum instead.
Answering your question, Dr. Muhammad Salama, PhD in Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu) states:
The intimate relation usually takes minutes not hours. If it is difficult for you to have bath at night, you may have this intimate relation after Fajr Prayer. In this way, you will have a lot of time before Zhuhr Prayer and you will take the bath in the morning.
Still, you should allow your husband to have an intimate relation with you at night because this is his right and your refusal will leave negative effects on him as well as your marital life.
As washing your hair at that time causes health problems for you, you can wash other parts of your body and then make tayammum (ritual using of clean dust to wipe it over one’s hands and face) instead of washing your head.
Allah Almighty knows best.