Heavier pellets would normally be the way to go because it creates more back pressure and slows the piston down. Fit also affects back pressure. Unfortunately a loose fitting 10.34 JSB may not create as much back pressure as a tight fitting 8.64 FTT. If you have other good fitting heavy pellets. Yes go with them because they will be less likely to detonate and cause damage. They will still diesel but dieseling isn't nearly as bad detonation.
Excessive dieseling can still burn through piston seals. This has been an issue with a few Weihrauchs the last couple of years.
Detonation (sounds like a gun shot) will likely rupture or burn through a factory Weihrauch seal in a few shots depending on severity. It can also shorten spring life significantly.
Cleaning the oil from the transfer port and barrel can help. The majority of the detonation is the lube burning that is trapped in the seal and the brazed seam in the end of the compression tube. The best way to address it is to break down the gun and mop out the lube and lube it properly.
At that point you can check the seal for damage. It's not uncommon for the seal to be scored by the cocking slot during assembly. That can be a source of lube getting into the compression chamber. A gun with a damaged seal will likely never clear up. The cocking slot needs to be deburred and the seal replaced.
If you don't have the technical skills or willingness to do the work yourself. You can keep shooting better fitting and good fitting heavy pellets to see if it clears up. If it doesn't you can send the gun back for warranty or have someone experienced work on it for you.
Pictures are of a recent new Hw30 piston seal. In one picture you can see the two parallel grooves cut in the seal by the cocking slot edges. The other picture shows the rupture out the side of the seal from the detonation. A chronograph can help assess damage like this.
