Thanks for your interest!
1. I've used POST request: https://github.com/DmitryKey/bert-solr-search/blob/master/src/client/solr_client.py#L85
2. Yeah, I recommend you to fork the repo on GitHub, clone it and start changing the Solr versions one by one from 6.6.0 all the way up to 8.2. This way you will notice smaller changes in the Solr API and should be able to fix them in code. Usually, I'd also read the Solr / Lucene changelog like this one for version 7.0: https://lucene.apache.org/solr/7_0_0//changes/Changes.html