Hi!
First of all you must set the transmitter values to 100% (or more) ! That is very important! Why? Because thats means you use as much servor gear travel as possible which means you minimize servo gear play. And servo gear play should always be recuced as much as possible to get a nice flying model.

Then ...Al trim settings should be done mechanicly first and not electronicly with the transmitter!
Why you ask?? Because that way you understand the importance of using as short servo arms as possible on especially elevator (one very important rule to remember) and how to set up the linkage so that there is no play.

Then when you have flown the plane once, land and mechanicly retrim the plane. Fly it a second time and notice how the plane feels. Land again and first then use the trim funktions in the transmitter. But be aware that reducing servo travel electronicly too much(reducing travel percentage in transmitter under 50%-75%) on especially elevator and aileron is not a good thing as it will induce servo gear play.
At home mechanicly re-trim the plane again and set all travels to 100% or more in the transmitter and fly it again!
That's the way you set up a plane!