Just to avoid misunderstandings. When you write
Please note that RichMemo package should not be dependent on DB package.
Thus if you create TDBRichMemo it should be in a separate package.
you mean completely different version lines.
I am not allowed to include Richmemo in uses clauses in DBRichMemo; but it is OK for you to let me work further on your current RichMemo present date?

For me to see there are two ways to go from here?
1. I include Richmemo in the uses clause of DBRichMemo
2. I take a copy of your Richmemo and rename it to DBRichmemo and make sure that both can be compiled and linked into Lazarus IDE.