I wouldn't go for a trailer, having tried it. Your kid is also already too big for a front seat. I have my kid in a seat on a step through bike. May I recommend the Copilot brand. There are several models but they share their sturdy unpretentiousness. However, the cargo problems with the seat are valid. I think a front rack would go pretty well, or maybe a bike packing style frame bag. There are some more expensive options (like long tail bikes or cargo bikes). These might last you a few more years.