Using the 99 trial as an example:
Start in a corner. Look at one of the sides. Instead of looking from the swords to your column, look from your column to where the swords are. If you see a sword in the column connected to yours, move one tile to the left/right. Repeat this again. If you've moved twice, you are now safe. If you don't see a sword on the first two tries, you are safe. Turn to face the other set of swords and repeat. If you can solve one side in time, then you'll at most get hit by one sword, and not the overlap of two, and you'll live.
Mechanics like this, that focus on parsing visual information, are intended to feel a little overwhelming, but then get easier as your brain works out a pattern to the chaos and you start to notice similarities. I would genuinely say these fights have very little visual clutter if you have the experience to know what you should and should not look at. FFXIV in general could be a lot better at teaching its playerbase, and I think we are seeing the fruits of that here and there. I think I'd respect the people begging for changes a bit more if they more correctly identified that the game was lacking in the way it teaches the player rather than the content itself being bad.
Generally speaking, you aren't going to instantly die to casual content unless you fail it catastrophically. If you stop focusing on doing it perfectly, and instead try to at least not fail it so badly that you'll die, you'll get faster at the bit you can handle, which gives you more time to do the bits you struggle more with.
And for better or worse, this game isn't about individual performance and party composition any more. Testing party composition just enforces a rigid meta that sucks for anyone playing a non-meta job (see: heavensward). Also, even if the devs did something like visual clutter to overwhelm you to induce sporadic party damage... well... that's the final boss of Zot, and it is probably overwhelming as hell to casual players since it even throws myself off as someone who does ultimate raids.
The only other skills to test are dps (requires enrage timers), group coordination (would not work in casual content at all), random flexing (likely problematic in casual content, as this stuff is often the hardest stuff in savage/ultimate), and probably some other stuff. So yeah, the devs don't have many other options for casual content, but IMO you have enough time, you just may need to accept imperfections at first until comfort and familiarity brings with it speed.