Tim Watts wrote:

My first thought was biscuit jointer, but they're not cheap for
something I will use very sparingly.

I got a cheapo biscuit jointer from LidAldi, and have used it on a few
extra jobs to the one it was bought for, but of course you can't buy
their tools when you need them ... but if you want expensive get a Domino.

I am wondering if a good dowelling jig might be a better investment?

My metal dowel jig hasn't had much use, the last time I used dowels I
wanted 15mm, so had to make a custom guide anyway.