got a lfr charm of the rising tide(885). i simmed it out comparing to the one i have. and mine was better. am not rly good at simcraft though. does it take in that you can use it at highe stacks in voidform? and is it even good.
So its probably not the best way to think of this trinket, but I got a 915 version and I worked it out that if used on cooldown it would be the equivalent of +1048 haste when averaged over 90 seconds. You are right to say it's value increases when saved to burst toward the end of a VF, however for me the math gets more complex there because if you're using it in those specific times, you're not using it on cooldown and its sitting there being nothing more than a +int stat stick with no secondary stats.
Add into the complexity of timing the trinket correctly with the added requirement that in order to get full use of the trinket you have to be standing still, I'd say the value of this trinket is situational. If you're really good about it it probably can do decent job for you, but unless you're really on the ball about it you're better served by a simple int/haste stat stick of decent ilvl.
It will sim higher as well as the default for sims is stationary (or at least significantly more still), so do keep that in mind as if you have to move you lose value (even if you gain the effect of the stack later (10stacks for a 20sec duration) that would be a stack gained at let's say second 12 to make 10 stacks as opposed to stack 10 at second 10 (assuming the trinket functions at 1stack per 1sec standing), something that sims would not show precisely. I would suggest checking how good you are at staying still during a fight/predicting 10-20seconds you can stay still for (it is possible on most fights).
Cheers.
Don't sim yourself as a shadow priest, default simcraft is not even close to being accurate. Go to h2p discord and look at pins.
Regarding your problem, tide is very slightly better than your trinkets, if you manage to get the full buff (i.e. 10 seconds of buff). Even at 8 seconds, it's worse. And obviously that assumes you use it at the right time to extend your voidform.