The fact that it's a 2007 domain changes this. you didn't start with a completely new domain, so I would check what happened on that domain before you bought it.
was this an expired domain you re registered, or did you buy it as a running site? that changes what I would look at.
Either way, pull it up on the Wayback Machine and see what it was used for before your content went up. If it was something totally different, or already had a history of poor quality content, that's worth knowing.
I would also check the links report in GSC and look at the old backlink profile. If there is a lot of spam from before you bought the domain, I would want to know that before assuming the 20k pages are simply a crawl budget problem.
A homepage indexing while almost nothing else does could still mean either thing, a domain trust issue from before, or a straightforward crawl priority problem. I would inspect a sample of the unindexed URLs in gsc and check their canonical, indexability, and whether Google sees them as valuable enough to index. If everything looks clean, the domain’s past is probably the reason.