I hear your concern. Sometimes complexity is good, though. We designed and optimized the classify-sklearn method to work "out of the box" so that you can use the default settings and get reasonable results without the need for a user to adjust the settings or intimately understand how it works.
Still, I get it if you'd rather use something you have better control over. In that case I recommend using the classify-consensus-vsearch method, which is just much better than classify-consensus-blast, and has more features to give you more control.
See here for more description of all of these methods, and benchmarking results:
Yes, on the same data resources page where the pretrained classifiers are found.
But I recommend waiting until next week when the next release of QIIME 2 comes out, and then take a look at the 2020.6 release data resources. The SILVA database files were built in a better way, using RESCRIPt