Any tips on very shy cats?

So we have Stewie (age 4), british shorthair, more human than cat. (he was taken from his mum at 4 weeks and raised by humans ). We took him in at bout 5-6 months old.

We have Dumpling (age 3), loves me and is okay with hubby. 50% british shorthair and 50% "homeless street kitty".

Then we have Polly and Sybil (age 2). Same mum as Dumpling but different "street kitty" dad. All 3 (Dumpling, Sybil and Polly) seem to be slightly skittish.

Sybil loves me too and seems okay with hubby, but Polly is very skittish.
She has got better over this past year, I can pick her up for short periods of time, and I can pet her for a minute or 2. But she is quite scared of hubby.

I try and spend one - one time serveral times a day with her, for short periods of time. And she is getting better. She will now take a treat from my hand.

She's in season almost constant (every 2-3weeks), and it is very difficult as she seems more skittish at these times. We have her sterilisation planned for this month. and Sybil will follow soon after, as we have no desire of having kittens.

I was just wondering if anyone has any other tips on taming a (very) skittish cat?
I don't mind if this is just her personality, and i'd accept it. She's great with the other cats, as Stewie seems to be her favourite (she uses him as a pillow, when she sleeps )

But maybe I could try something else to make her feel more at ease with "us humans". Tips?