About hum suppression:
unfortunately most of the hum noise is NOT 50 or 60Hz (depending on where you live), but also harmonics of this base frequency, that is 100 Hz, 150 Hz, 200 Hz etc.
So your low cut filter will not solve the problem.
There‘s several strategies you can use/combine:
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2 single coils wired antiparallel -> this is known as humbucker. The traditional humbucker dampens high frequencies. There are also humbuckers available in single coil size. They have reduced the high damping problem a lot and sound like single coils. They are called noise free single coils or similar.
There are even more complex circuits for combining the advantages of single coil and humbucker, but this would lead too far here
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if you play a crunch or distorted sound, then the hum is actually hardly or not audible anymore. It only becomes audible once the tone fades out. You can now suppress the hum with a noise gate.
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if the hum gets introduced in the cable rather than in the pickup, you can put a little high impedance amplifier into the guitar. Any old amplifier will NOT do, it must be an amplifer suitable for high impedance sources like a guitar pickup. Actually it‘s not even an amplifier in the strict sense of the word, it‘s an impedance converter.
Shielding the electronics compartment of the guitar with aluminum foil also helps. -
using high quality cable also helps.
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if you REALLY insist on solving the hum problem with filters, then you need several very narrow notch filters for the base frequency and all the harmonics. This means you will have to pass your signal through a low latency high power audio DSP. Low latency, because high latency will affect the playing of the guitar. And the sound of the guitar will change. And not cheap.
About your high frequency cut filter:
Allthough you are right about the BASE frequency of your D6 note, the tones produced by the guitar strings also include higher frequencies, the harmonics. If you cut these frequencies, your guitar will sound very dull. The pickups can put out frequencies up to 15kHz, depending on type of pickup, load etc. Expanding this topic here might be too much for now.
About sustain:
This is what sustain/compressor effect pedals are for. Of course you can try to build your own digital or analog effect pedals, but as mentioned above, this is no job for the arduino, and also no job for someone with no experience in the field. If you want to learn and build it yourself, then by all means go ahead. But you will not save any money, if that is your goal. The commercially available effect pedals are cheaper and better than anything you could build yourself on the first attempt.
Advice from someone who has been playing guitar and building audio electronics for 20 years
Thomas