I'd get a bigger childseat and then a tagalong, or a trailer - all available secondhand. Weeho's tremendously expensive in comparison and limited use. Also much longer than a tagalong, so tricky around corners/bollards.
One thing I've noticed whenever we're shown pics of 'european cycling' is that they have kids in bike seats much older/bigger than we would here. My daughter who is big for her age was fine in copilot until 4 1/2 - she'd still fit in it okay now if needed, and she's the size of a well-grown six year old. She wouldn't fit a hamax though. The simple european seats seem good for bigger kids -- well up to school age.
When I was at same stage as you, I thought the wee-hoo would be a great option. But looking back at it, very glad I didn't go down that route. (I did worry about her being used to falling asleep on the tandem or tagalong, but she's never even come close to it.)