I am in a similar situation as kamchatsky,
Was planning to go back to HK at the end of 2019, but covid started. So now planning to go back to HK and mainland China to visit mother in-laws.

My HK ID was issued in 2007 and the home return permit has now expired, at the moment, my HK ID card is still valid as I have checked, just need to renew it.

Upon searching HK Immigration FAQ once you arrive in HK, you need to renew your HK ID card within 30 days. So need to pre-book this.
While waiting for my HK ID card renewal, I would like to travel to China. The question is can I renew my Home Return Permit to mainland China with the old HK ID card and can I prebook this online and where?