All the problems are because of the same root cause. They played iso ball, a side effect is a lot more wear and tear because essentially only 2 people are playing even though there are 5 players on the court.
Not sure if you were watching the games last year, but that's not quite correct.
Last year both Kyle and Demar were injured for significant portions of the regular season, so the other star played much heavier minutes to compensate - which in itself can lead that remaining player to wear out. Demar was injured at the start of the season and then again midway by a freak ankle sprain, so Kyle really shouldered a lot. It was actually right around this time last year that Kyle had surgery and was out for around 6 weeks until the playoffs essentially began, so Demar stepped up. Alot of this was due to, out of the 5 starters on the floor, 2 of them were shooting very poorly in Patterson and Carroll - especially in the playoffs which increased the usage rate of Kyle and Demar to compensate. They also had essentially four players who were on their first season of NBA playing time in Siakam, VanVleet, Poeltl and Wright. Valanciunas was playing a lot of the traditional big man game with his back to the basket and he was having a tough time closing out on opposing bigs who shot the 3ball, so Casey elected to sit him rather than expose them defensively.
This year they got rid of Carroll, Patterson, Joseph and brought in 3 point specialist CJ Miles and drafted defensive stalwart OG Anunoby. VanVleet is absolutely ripping it up right now, Poeltl is an awesome backup to JV, Siakam has improved immensely from last year and Delon finally has a full season of NBA experience and is injury free to showcase the eurostep. JV and Demar are now spreading the floor by adding in frequent 3 point makes and JV does a much better job of closing out.
Casey now has two defined 5 man units which is a huge change from last year where he would always keep Kyle or Demar on the court with the 2nd unit, because of their lack of experience and productivity. He has never played 10 players so often, because he was never able to - now they are all playing well defensively and offensively, except for Lucas and Norman who either ride the bench or are injury replacements.
To the casual observer, it might seem like it's just a switch from ISO ball to a balanced attack, but it's much more than that.
One major problem with the current team is that they seem to lose a lot of close games. They may have a lot of blowout victories, but they lose too many close games right now and the playoffs are going to be much more about close games that they need to be able to execute.