No. A simple website won't help, it needs to be a Lemmy instance. Moreover, it needs to be a federated one.
And then, that "invisible" data being available to other admins, is a problem with federation, not with Lemmy.
Now, there could very well be efforts made to make the cleartext data of each instance users available only to the admins of that instance (and only share aggregated data with other instances), but that would also require a lot more consideration wrt mutual instance trust in the network.
Right now, since votes and other actions are public (to the federated instances admins anyway), it is doable to detect and assert foul play. The downside of this is that it allows abusers to malevolently collect data and do the same bad things that you are so certain the alternatives to Lemmy don't do (yeah, as if).
If the instances shared only aggregated data with one another, it would be much harder for abusive small instance owners to spy on any user on the network (still possible, but it would essentially would be as hard as for anyone else, as it would involve heuristics and lots of intelligence, to interpolate the missing information); but it would also be much harder for legit admins trying to enforce moderation to inspect what happened on federated instances. They would have to take those instance's admins at their words.
As an additional note: that "invisible" data that other platforms allegedly don't share, is for sale. That's what surveillance capitalism is all about... At least with Lemmy, the barrier of entry to get our data is "federation", not "money".
Edit: WTF bro, a day and a half before writing this wrong comment I'm answering to, you wrote a properly worded, technically correct (top level) comment... Were you half asleep on this one??
Edit 2: nah, the reason why your other comment was technically correct and properly worded is that you stole it (would have been so easy to give credit...) SMH. 😮💨
Edit 3: So I checked your comment history (after seeing that other comment of yours about the user that mass downvoted you, I was legit curious how bad it could have been), you seem technically knowledgeable, and also educated. Thus, I reiterate, this specific comment, what gives!?