Everyone has their own idea about renting out their cali. Here's mine: When we had our new T6.1 in April 2022, we still had our old trusty t4 westfalia california and then I also thought about renting out via Goboony. You don't have to worry about the insurance (at least with us, maybe different with you): you take an extra day insurance that the customer pays. Do check whether your own insurer agrees with the rental (usually yes, but sometimes not or your policy will be higher). You can have it cleaned by the renters before returning it and if it is not ok you can (if you indicated this in advance) ask for a fee or: immediately charge for all your own cleaning and do it yourself. Actually it is a nobrainer as someone here said, but after having two campervans for more than half a year, and a lot of contact with goboony and with people who rent out a small campervan, I decided to sell the t4 anyway: it seemed to me to take too much time on delivery, receiving it back and checking if it’s ok and that in periods when we ourselves will be traveling. So it's going to be difficult. Also: you know your van but renters not: they will make do things fault and mat break stuff. You can earn a reasonable amount of money, but it will not be as much as you imagine (an old bus or the youngest cali: they have the highest daily prices, everything in between ... is sometimes clearly less.
At the end of the day, it’s your cali so you must decide whether you do it or not.