- WD is by far the best solo class, at least in the beginning. You'll be able to hit level 70 about 1 hour or more earlier than any other class if you do kill streaks instead of rifts. Drawback: You'll be starved for gold, mats, and gear once you hit 70. I suggest to do lots of boss bounties and occasional rifts in between to make sure you have approx. 1m gold when you hit 70 (which is what you need). Build? Doesn't really matter, key is one spell: Haunt. Before getting any 6 piece set, all spells for all classes suck. The game is about multiplicators, and the 6 piece bonus of all sets does that, it multiplies your damage by a factor of ~20. BUT the WD spell "Haunt" deals so insane damage that it's very very powerful before you get any of the sets, so you can just pick 5 other spells to use as well while you spam haunt. For exact leveling specs Google "WD 1-70 leveling guide" or something like that, there'll be a thousand. Also take a look at our frontpage and forum in the week before S9 starts; there's usually lots of guides being posted closer to season start.
- T6 breakpoint is simply when you get the 6 piece set. Without it, you'll run ~T1 for some gear and keys, then 1 rift T4 to unlock the GR20, then clear the GR20, do the bosses in the season journey to get the free set... and once you have the free set, you jump right into T6-T8. You'll then steadily progress higher and higher to approx. T11, and there you'll stay. No one, I repeat NO ONE in their right mind runs T13. Even high-end players with best possible gear oftentimes run T11 because T12/T13 are just so much slower with a minuscule gain in what you get out of it. Essentially, you always want to be able to almost one-shot elites (or kill them in 2-3 seconds). If it takes longer, go lower.
- Level-up must-have skill: Haunt. There are tons of level-up guides as I said if you Google. I don't want to post one particular here as I don't want to favor one or the other; tons of people put work into it, and I can't post them all, so I'll rather post none. (I would just have to Google as well, to be honest ;)). I've started WD so often that I know pretty much by heart what to do.
- Try to group up with a DH and monk, if you have the choice. Fourth class wizard or barb. DH is just absolutely insanely fast at T6-T11 rifts. Monks the same, very fast, but also versatile as they can go support (give you more survivability) when things become too difficult. Wizard is a bit slower in the beginning but adds insane DPS a bit later, let's say 15-20 hours into the season when other classes start to struggle. That is, if your group stays together that long (which is a lot of fun!). Key is that you always want to clear as many rifts as fast as possible; best is an average of 2-4 minutes, not more. If your rifts take 5+ minutes it's fun, but you're slowing down your progress. We usually push for 2-3 hours but then go back to speed.
- RE: tips for conquest progression and free set: not sure what you mean here. Free set? Finish chapter 4. Conquests? Different story.
I'll update my Season Journey Tracker (see signature) once S8 has ended. That'll give you an idea what you need to do to get the free sets.