Do you intend to restart this thread as a Sultai-RNA edition? I think that's cleaner than modifying an old thread. Nothing wrong with copy-pasting parts that you still consider to be true.
I think my next project should be the sideboard for this deck. It's my favorite and I certainly have more than 60 cards for it, but I really lack ideas other than "fill it with Duress/Negate to swap for all the removal cards".
Haven't played a single Bo3 match yet, but this deck kind of screams for it. Even the pros write that you're prone to lose game1 vs either end of the spectrum
For sideboard these are some of the cards to consider, though it depends also if you're running the explore package or going maindeck Thief (I put a number in just so it'll show links):
Disdainful Stroke
1 x
Negate1 x
Duress1 x
Thief of Sanity1 x
Kraul Harpooner1 x
Unmoored Ego1 x
Crushing Canopy1 x
Eldest Reborn1 x
Midnight ReaperAlso any extras of things you have main like Contempt (some lists only have contempt in the side though). I run maindeck Reapers which I probably shouldn't since though it's great in midrange mirrors that's not what MTGA is about right now. Obviously you don't run all of these cards, though I think 4 duress, 1 d-stroke, 2 negate is probably a decent minimum.
I think the explore list is the best version, but there's reasons you might want to go in another direction for sure.
For bad matchups the ones that stand out are mono blue aggro and nexus of gates. Mono blue is just too efficient at stopping your expensive plays and dive down is pretty absurd. Wildgrowth Walker is huge in this, but it can be denied by trickster or all your creatures can be countered. Harpooner out of the side is a big deal here and can make a real difference.
Nexus of Gates is close to unbeatable if I go by my record against it, pre sideboard they outvalue hard and can just win from nowhere while post sideboard they can almost be playing a different deck if the choose to and if you guess wrong it's bad times.