Well yeah Overkill is using the traditional crossover/thrash sound on Infectious, while Megadeth's sweating bullets is not what would be considered traditional thrash based on its sound - it has a more heavy metal vibe. I also wouldn't say that sweating bullets is typical of megadeth's sound - its more of an outlier imo. I wouldnt say that it doesnt have a basic pattern though - it has a very simple pattern/structure, but the guitar and bass are absent outside of the solo and chorus sections.
Gotcha.
This is all semantics at this point , but an 'instrumental' is literally a piece of music without vocals. A 'solo' is when a single musician or instrument plays a melody either by itself, with all of the other musician's quiet, or over the rhythm being played by the other musicians. aka you can have a solo on an instrumental track.
Hmm. Well, based on the examples you posted, I think you'd mainly like heavy metal (a totally different sound than thrash metal) and power metal or simply just certain hard rock bands. Most doom metal is very different than what you've posted. The more bluesy doom rock/metal is somewhat in the same region as heavy metal, but the really heavy stoner/doom metal or funeral doom is much different. Sludge doesnt really fit either. A lot of sludge bands use screaming or very 'harsh' vocals and very muddy tone/sound (hence the name). Gothic or symphonic stuff might fit, but you didn't link any.
I'd suggest looking through some advanced search results on encyclopedia metallum:
search #1:
http://www.metal-archives.com/search/advanced/searching/albums?bandName=&releaseTitle=&releaseYearFrom=2000&releaseMonthFrom=01&releaseYearTo=2017&releaseMonthTo=01&country=&location=&releaseLabelName=&releaseCatalogNumber=&releaseIdentifiers=&releaseRecordingInfo=&releaseDescription=&releaseNotes=&genre=power%2Fheavy&releaseType%5B%5D=10#albums
- genre: power/heavy
- only bands that have compilation albums (my thought here is that it will limit things to more established bands. This avoids hundreds of bands who only have 1 album, etc)
- compilation album released between 2000 and 2017
search #2:
http://www.metal-archives.com/search/advanced/searching/albums?bandName=&releaseTitle=&releaseYearFrom=2000&releaseMonthFrom=01&releaseYearTo=2017&releaseMonthTo=01&country=&location=&releaseLabelName=&releaseCatalogNumber=&releaseIdentifiers=&releaseRecordingInfo=&releaseDescription=&releaseNotes=&genre=power%2Fsymphonic&releaseType%5B%5D=10#albums
Same thing but with power/symphonic genre.
These searches can be further defined with country of origin, etc if you want. Hope these help!