Reward Cards - worth applying now? new homeowner

Zogger

Hi,

I'd like to get a reward / cashback card for general spending and pay off in full each month.
I took out a first mortgage and became a homeowner in mid November, and was added to the electoral register in January. I was only at my previous rented address for 6 months, with 4 years at the previous rented address to that. Age 25

Salary including bonus is fairly near the higher rate tax band

I have had one credit card that I've held since 2006, with a credit limit of £500, always paid on time with no late payments and almost always in full, the reported balance in Noddle for December was £388 though due to a big trip to ikea to furnish the new house.
Don't have any other missed payments, ccjs or whatnot

A couple of weeks ago tried to apply for a creation cashback card but was declined online with no indication as to why. I also tried to apply for the Halifax rewards current account online a few weeks earlier, but was told I would only be able to apply in branch, no indication why again, so I didn't bother.
I've requested the statutory Equifax report to find out if they have me on the electoral register.

In the meantime, if they do have me on the electoral register, would it be worth trying to apply for another reward card? I'm considering the amazon mastercard as I shop a fair amount at amazon anyway and the 1% / 2% back in vouchers is very competitive, but not sure if I would be accepted. Would I be wasting my time doing this and better off waiting for a while?

Thanks