Hi @JoaoGabrielMoraes,
Quality scores of 20+ are generally considered good quality. 30 is high yes, but that doesn't necessarily mean you need restrict yourself to that setting.
This has actually been changed to 12 nts in more recent version of q2-dada2.
We need more info to be able to suggest anything here because this really depends on the overlap region of your amplicons. Search again through the forum, there are lots of examples of how this overlap region is calculated, and other considerations you should be aware of. Ex here.
While it is generally a good idea to keep only high quality reads for downstream analyses (garbage in, garbage out) we have come a long way on relying so heavily on quality scores. In your case I would think you are perhaps being a bit too strict, and you could eye ball the median scores of 20 as a starting point. Obviously you can truncate more if you can afford it based on your overlap region, but I think you can certainly get away with a less strict cut off.