Solr: indexing nested JSON files + some fields independent of UniqueKey (need new core?) - indexing

Question

I am working on an NLP project and I have a large amount of text data to index with Solr. I have already created an initial index (Solr core) with fields title, authors, publication date, authors, abstract. The is an ID that is unique to each article (PMID). Since then, I have extracted more information from the dataset and I am stuck with how to incorporate this new info into the existing index. I don't know how to approach the problem and I would appreciate suggestions.
The new information is currently stored in JSON files that look like this:
{id: {entity: [[33, 39, 0, subj], [103, 115, 1, obj], ...],
another_entity: [[88, 95, 0, subj], [444, 449, 1, obj], ...],
...},
another id,
...}
where the integers are the character span and the index of the sentence the entity appears in.
Is there a way to have something like subfields in Solr? Since the id is the same as the unique key in the main index I was thinking of adding a field entities, but then this field would need to have its own subfields start character, end character, sentence index, dependency tag. I have come across Nested Child Documents and I am considering changing the structure of the extracted information to:
{id: {entity: [{start:33, end:39, sent_idx:0, dep_tag:'subj'},
{start:103, end:115, sent_idx:1, dep_tag:'obj'}, ...],
another_entity: [{}, {}, ...],
...},
another id,
...}
Having keys for the nested values, I should be able to use the methods linked above - though I am still unsure if I am on the right track here. Is there a better way to approach this? All fields should be searchable. I am familiar with Python, and so far I have been using the library subprocess to post documents to Solr via Python script
sp.Popen(f"./post -c {core_name} {json_path}", shell=True, cwd=SOLR_BIN_DIR)
Additionally, I want to index some information that is not linked to a specific PMID (does not have the same unique key), so I assume I need to create a new Solr core for it? Does it mean I have to switch to SolrCloud mode? So far I have been using a simple, single core.
Example of such information (abbreviations and the respective long form - also stored in a JSON file):
{"IEOP": "immunoelectroosmophoresis",
"ELISA": "enzyme-linked immunosorbent assay",
"GAGs": "glycosaminoglycans",
...}
I would appreciate any input - thank you!
S.