non-EEA family member -visa

[AH84]

Hi everyone,
I would really appreciate some advice on the issue I've got.
So I hold 2 citizenships one is EEA (LAtvia) and 2nd is British Citizenship.
My mum lives in Latvia and has non-citizen passport of LAtvia, which means she needs visa to travel to the UK.
In the last 10yrs she was always getting a visitor visa. She does not stay longer then 6 month, but there is a possibility she may wish to stay longer in the future.
I've got few questions in my head. Could we apply for EEA family member residence permit or we can't b/c I'm now also British citizen? Also if she gets EEA family member permit she can't be out of the UK for more then 6 month in a year is that right?
We are thinking to get 10 years visitor visa. Not sure if could try anything else.
I thought as a British citizen you would have more rights for parents to visit, but it looks like it's more difficult.
I was also thinking about ''Surinder Singh'' application form.
Many thanks in advance!