Hi there,

for an unique lightsaber crystal I am looking for some advice how to get a legal homebrown build (not too weak or too strong). The crystal will belong to the Shard species.

Thanks in advance!

There are no official rules for custom crystals. But there are so many official crystals you could repurpose one of them.

From looking at the official options, here's how most of the crystals seem to be set up:
You start from the standard stats (Dam 6, Crit 2, Breach 1, Sunder) and add 1-2 base modifications and 2-3 optional modifications from the following list (some of them may be chosen multiple times):

• Damage +1
• Crit -1
• 1 rank in any of these qualities: Burn, Concussive (careful, these two are more powerful than the other options), Defensive, Deflection, Disorient, Vicious
• 1 innate rank in a ranked lower- to mid-tier talent
• a custom effect of your choice (possibly interacting with a certain talent or Force power; this is where you can personalize the crystal to whomever will be using it)

While I haven't seen it during my (admittedly short) research, providing a rank in a general skill is also feasible.
Another option to consider is to weaken the crystal in one aspect (Damage -1, Crit +1, remove Sunder or replace Breach 1 with Pierce X) and thus allow an additional modification option.