How to Trakt + Last.fm scrobble only for my own user? What about other users history?

I cant comment on Last FM as I don’t use it.

With regards to trakt well plugins are dead, the trakt plugin partially still works but probably sooner rather than later it will not do anything.
I have no idea if its still operational but with the trakt plugin you can configure other accounts with this Configuration - trakt (for Plex)

However you can also use webhooks for trakt.
Even then there are two options.
https://plaxt.astandke.com/ If I recall it requires that you have a PlexPass… and you do.
You can create a webhook for any plex account and scrobble it to any trakt account. I seem to remember the other users do not need to have a PlexPass.

The other option for webhooks is the official Trakt one. However for every account you have to have both PlexPass and Trakt VIP. So probably not viable for everyone or indeed hardly anyone with multiple users.

Finally the downside of webhooks. They are one way. Anything you do in Plex will get pushed to Trakt. There is currently no way to pull from Trakt to Plex with webhooks.

There is a feature request for official Trakt support built in… but I certainly won’t be holding my breath. The higher echelons of Plex where very dismissive when they announced plugins were being removed and Trakt was explicitly mentioned.
Still every vote counts (or not)