Mickey
I don’t know where in the country you are, but I have two butts rigged up in a similar situation to yours and to be honest they overflow in winter but don’t fill up quickly enough in summer. I don’t get anywhere near enough water from the butts to be able to water the garden, so I’ve tended to use it to water plants in modules, small pots as well as freshly sown seeds/transplants. I got them originally to use for tomatoes and I still water them from the butts up to final pot stage, but if you want to water gardens in the SE of England, you’d need butts containing up to 1000 gallons, I think.
If you’re in a wetter part of the country you may be in a better situation………