Looks great. Hope the glue is sturdy enough. If your toddler isn’t to hard on it this will work too :)

Added it for saftey reasons

I’d also go for the four fins. Easier to do and more robust, since you can prepare two V shaped double fins, put them together outside and the slide them into the main body together.

Also pretty cool to play with: a bottle rocket. Take a regular plastic bottle, turn it upside down. Add fins to make it stand. Attach a manual bicycle pump to the bottleneck. Fill the bottle a little bit with water and pump it up until the pressure releases the bottom seal.

Edit: make sure that the bottom seal can eventually slide off to release the rocket.

My 5min search resulted in this guide on it: https://www.science-sparks.com/making-a-bottle-rocket/

As cool as it looks you may want to consider dulling the tip , depending on the age and body control of your child.

For the fins I’d just use a hand saw to make slits and glue them right in. Harder to break them off. For four just cut a cross in the bottom. For three cut from the sides.

Absolutely, the material cost for such a system is about 7-10k strongly dependent on the battery. The other ~10k is for setting stuff up (which takes about 1-2days).

The small balcony systems can be setup by pretty much everyone. They also have the best ROI compared to bigger energy producers.

A reasonable sized solar power system with around 10kWp and 10kWh battery costs at least 16k€ in Germany when done by a professional. Depending on your power consumption in general this can be a bad investment even with a heat pump. Therefore I would separate this from the costs of a hwat pump, since it is not strictly required. Especially since you can get energy plans selling power for ~26ct/kWh.