Roland GK-KIT-BG3 Bass

The GK-3b system can be used on most basses to add a hexaphonic pickup to your bass, which enables you to connect to devices with a GK-input (like Roland GK guitar synthesizers and effect units). The installation kit offers the same functionality as the outboard GK-system, with program change and source switches as well as Synth Volume. Takes a little installation effort, but any guitar technician should be able to install this system, and you may do it yourself as well, if you are a little handy with tools. Using the installation kit makes the
When installed and adjusted correctly, the unit provides great string-by-string tracking and subsequent MIDI tracking. Note that the unit itself does not output MIDI signals, and requires the use of a converter (such as the mentioned Roland GK-synthesizers or similar). Also note that tracking below an open A-string will suffer from a bit of latency due to the time required for pitch detection. All in all, the Roland GK unit with a compatible converter or synth device is an excellent entry point to the world of MIDI bass.

If you are considering installing this instead of the external one, bear in mind:

As a bass player, you still need to change strings for loose vibration to track better but just enough it doesn't catch any fret buzz, clicks and clacks.

I settled on the DIY pickup kit because it was the cheaper option. Took me a while to measure, drill holes, hide it to an active bass and get this signal sent to Roland GR55 successfully.

It gives you loads of options and sustainability over the external pickup since it's a split signal and doesn't affect your clean tone since you are going to use an extra proprietary cable. It's not a regular 5-pin MIDI OUT, but a 13-pin that only goes to Roland devices. However, you can pretty much send it literally any synthesizer in the chain with MIDI IN.

Your guitar cavity needs to have enough free space for a Raspberry Pi sized circuit board.

No need to learn japanese to understand manual, just use the circuit diagram.

Manual has both active and passive bass diagram. I might even try to put one on a passive bass in future.