I’ve looked at my banks (Monzo and Starling) but neither provides Cash ISA with partial transfers.
You’ve both been very helpful in getting /me/ to understand what I want, so thank you.
I will probably just top up Freetrade and invest it only when I know I won’t need the cash, and open the other S&S ISA in April.
That new S&S ISA won’t take up all of my 2021/22 allowence, so I could put the rest of that allowence into another type of throwaway Cash ISA , and put /that/ into Freetrade April 2022, which won’t affect my 2022/23 allowence, assuming we live that long 
Thanks again, it’s good to talk.