How do you polietly say no to a company that wants you to stay?

I'm not very happy in my job and I have found myself a new one.

I'm working my 3 weeks notice.

Our CEO asked me if I would like to stay and get paid a bit more than I currently get here but less in my new job.

I'm leaving cause I'm tired of doing their cover when their own receptionists can't even do their own job.

In my new job I won't have to do this.

Should I just something like

"Thank you for the opportunity but I am declining the offer. All the best"

Does that sound ok?

Updates
1 mo
They wanted me to stay and help for their reception cover er.
I asked them six months a go for a pay rise for this reason and they said no to me.
Now I'm leaving they treat me differently.
They can find someone else
How do you polietly say no to a company that wants you to stay?
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