You have very valid strong reasons for wanting to name your daughter Dinah. I think your dh is being unreasonable. This obviously means a huge amount to you, and his objection to the name being old fashioned is much less significant than you having planned for decades to honour your mum. "Pensioners' names" are v fashionable now anyway, so his argument against doesn't stack up.
You don't need to name baby until around 40 days after birth, so you don't necessarily need to decide now. After our babies were born both dh and I were much more relaxed about the name choice, just having our lovely baby was enough, the name seemed far less important.
I do think the woman has the edge with name picking here with pregnancy, birth, breastfeeding, and naming the baby being the first big decision. Definitely a mutually chosen name is best by far, but in a complete stalemate where one party will be disappointed, I don't think it should be the person who has carried the baby and born all the responsibility up until that point.
If you really can't reach an agreement tbh that bodes poorly for raising a child together as there are an awful lot of decisions to make together....
Are you married?
Whose surname will baby have?
And Dinah is an absolutely beautiful name.
Not at all old fashioned.
Lisa is v dated.