I like Sigma lenses the most, Tamron close second. Samyang have good reviews, just with little bit of quality variability based on user experience here on forum.
While a personal preference. I would say when you know what focal length you want, it can not hurt to visit a local camera store or buy from a store with a return policy.
I wouldn't call Sigma's or Tamron's lenses cheap. They are less expensive, which is good, but perform as good as Sony lenses.
When I started with Sony, I kept two lenses from my previous Canon system: 85mm f/1.8 and 70-200 f/4 L. They perform very nicely.
Not necessarily, 3rd parties can't sell rear mounted TCs on E mount so your only or most appealing option in that regard would be the 70-200 f2.8 GM II which is not much smaller or lighter than a xx-400 but certainly has some advantages, for a price. The Tamron 70-180 f2.8 is great too but no TCs. For xx-400 there's the cheaper Sigma DN which isn't as well sealed and has weaker AF but looks optically fine, the Tamron 50-400, and the 100-400 GM which does take TCs.
It has been years now since I've used Nikon, and I've never used the Z7II so I really can't make any comparison, nor am I interested in Nikon's offerings any more to even bother. My Sony A1, A7R IV (now traded) and A7R V all have excellent ability to resolve the outstanding Sony and Voigtlander lenses I put on them, and dynamic range is very good. The A1 is better with noise than the a7R IV was or the A7R V, but it really is not a significant issue in any case.
Favorite and most used 3rd party lenses used on my a7iii,a7iv, and a7c, in order of most used.
