I mean, it depends on how much weight you want to add.

According to the internet, a penny weighs 2.5g or 0.088oz. A roll of pennies costs 50c and adds 125g or 4.4oz.

Some prints I’ve added just a few pennies to, and some a few rolls. It just depends what you need.

My solution has been pennies. They’re cheap and and easy to get. Since they are a fixed size, it’s really easy to model a cavity for them in your slicer. You can also leave them in their rolls for bigger projects.

My current printer is a MK3S+, and I love that Prusa is still pushing firmware updates for it. However, my personal experience with the machine has been pretty lackluster with multiple parts breaking and sub-par print quality. In fact, something else just broke yesterday (I have yet to diagnose exactly what) which is making me more eager to upgrade. I do wonder if I just got unlucky considering their reputation, but I can’t say I’m rushing back to then with enthusiasm.

I do agree with your general point about open source. For that reason, the Prusa, Voron, and Ratrig are the most intriguing options. However, if one of the other two are likely to provide a better experience, I think it’s silly to not even consider them.