Here are some basic notes on how I setup my Ender 5 with the above mentioned modifications. This is not a complete guide, just the key information and some relevant links. It might be a good idea to look for guides on YouTube and reading the official documentation.
Pick a recent LTS branch of Marlin.
Configure it similarly to what I've done here.
Compile it as explained in the Marlin documentation or README.
I use Visual Studio Code with the PlatformIO IDE extension, but there is also a Marlin auto build extension available.
You should be able to compile by pressing the checkmark and (when successful) then find the firmware.bin file in .pio/build/LPC1769/.
Copy that to the SD-card connected to the board and it will be flashed during the next startup.
Wire everything as explained in the manual.
To use sensorless homing: don't connect the X and Y endstop switches and don't cut off the DTAG pin of the stepper drivers.
Configuring the sensitivity takes some trial and error. Tune this with X_STALL_SENSITIVITY and Y_STALL_SENSITIVITY in Marlin/Configuration_adv.h. Try a value in the middle first, then go lower until it doesn't home correctly (loud noise as the stepper keeps trying to move while hitting the end) and then increase the value again.
Connect the BL-touch probe to the servo/probe ports as shown in the manual. You might need to switch a few wires around on the cable that you receive in the box.
Configure the offset of the probe to the nozzle with NOZZLE_TO_PROBE_OFFSET in Marlin/Configuration.h. You should just be able to move a piece of printer paper between the nozzle and bed. Also ensure that the bed physically leveled.
Especially if you don't have an original sensor (shown below), change the config accordingly and double check if 5V is supported.
