First: I was pointed to the Put tape on the 5V pin - Why and how topic. How does one reply to an existing topic? I logged in, I seem to be able to create new topics, but I can't find a "reply" button. Why is it hidden?
Second: I don't understand the issue. A 3D printer controller should use about 20mA to run the CPU, maybe about 100mA for the stepper drivers, and maybe another 100mA for the backlight on the display. About 200mA. If your rapsberry pi supply can't handle that, better to fix that now instead of wait for your powersupply to age and degrade a bit.
Lastly: I've made a little board that takes 12V from the printer's powersupply and makes a nice 5V for the pi. This would prevent the whole printer-before-pi and pi-before-printer problems. Also... I read something about fans running.... This little board has two small mosfets that can switch a fan. I have a fan on my hotend and a fan in my powersupply that would otherwise be 100% on all the time. So I've implemented for the hotend: "fan1_on = temp > 50 OR hotend_setpoint > 0. For the one in the powersupply I have fan2_on = bed_setpoint > 0 OR hotend_setpoint > 0. Works for me!
Anybody interested in testing my little boards? I just realized today that there are printers that run on 24V. The DCDC chip is not going to like that. So only if you have a 12V printer. I'm going to try to sell them after some more testing. For now a few "I'll charge you nothing" boards are available for testing.