The Magic novels are all on Amazon now as eBooks - definitely read the Brother's War, Planeswalker, and the Ice Age books!
The Brothers' War (Artifacts Cycle) https://www.amazon.com/dp/B07C91KLV8/ref=cm_sw_r_cp_apa_utHmBbP7EPNPZ
Amazon has a kindle version of Brother's War for 5 USD. Same for all of the Artifacts Cycle books, all at 5 USD. The paper novels vary wildly in price. The Onimbus books probably the cheapest way, totaling ~100 USD (these versions also are not available as kindle books via Amazon).
There is a kindle version of it: https://www.amazon.com/Brothers-War-Artifacts-Cycle-ebook/dp/B07C91KLV8/ref=tmm_kin_swatch_0?_encoding=UTF8&qid=&sr=
At least some of them are available on Kindle.
Like here for The Brothers War: https://www.amazon.com/dp/B07C91KLV8/ref=cm_sw_r_apan_9SMV2RJESR0C4DR6PQAS
I'm not sure that Saga's make sense because we're not "in the present thinking about Brothers War", we're in the Brothers War. I guess we might get 1+ Saga's related to the Thran, but don't expect them to be as prevelant as they were in Dominaria. I also expect Urza to get reprinted in Double Masters, so would want to stay as far away from that, or any other MH1 card, as possible (literally just sold my playset yesterday).
I'm still figuring out what, if anythign to go in on, but my general thoughts:
The Brothers' War Book & The Artifact Cycle Book 1. These aren't "real specs", but I think you'll get a lot of folks wanting to read the books and these could go up in price (if WOTC doesn't "reprint" them lol).
[[Mox Opal]], Secret Lair Artifact Lands, [[Treasure Vault]], [[Ugin, the Ineffable]] and [Sai, Master Thopterist]]. All of these seem like strong pick-ups if we get a "pushed" artifact set (albeit each of them for different reasons).
[[Goblin Welder]], [[Goblin Engineer]] and [[Emry, Lurker of the Loch]]. These are all super powerful, low cc artifact recursion engines and I can't believe they're as inexpensive as they are -- how Emry is a $1 card right now is beyond me.
[[Tezzeret, Betrayer of Flesh]]. I really like your callout of Tezz. He's a $3.5 card and has built in card advantage + cost reduction, which is a powerful combination. While it has a good interaction with [[Reckoner Bankbuster]], it feels like it was designed with a future set in mind. Anyway, agree this one looks great. I don't normally like Standard Specs, but this one seems good.
[[Yavimaya, Cradle of Growth]] and [[Nissa, Who Shakes the World]]. I actually think the existence of these two cards together alone is enough that they don't need anything (and will both go up over time). However, I would not be at all surprised to see some strong "forest" support cards in the set due to lore tie-in with Argoth and Urza/Yavimaya.
Side note, I have wanted WOTC to make an MTG live action on Brothers War for over a decade and I hope this is what finally makes it happen.
With the Dominaria set coming up later this year you can't go wrong with the Artifacts Cycle, starting with The Brother's War.
There's a great deal in the novels, there's all but the very first handful of novels on Amazon as Kindle editions. The Artifacts Cycle starting with The Brother's War is a good place to start, there are four novels in the cycle.
The art books for each of the planes contains a great deal of lore too for those planes they were published for. Aside from the books there are novellas in serialized form for most sets on the Magic Story page.