I would like to play Wastland 3 with a charater who throws grenades of all kinds as his main attack and does not use a firearm.

As a secondary "weapon" I was thinking more of mechanics and the robots there that you can use.
Or maybe other supporters like healing.
But the character should not be a healer, but primarily deal with throwing projectiles of all kinds.

Is there a guide for this, so that you can build good synergies together / look off ?

You can also build something useful yourself, but it's helpful if someone has already tried it out and advises something different, such as better not to be a mass murderer but rather a disciple of metal, etc. Guides are always very helpful^^

Thanks

Grenadier would face a few challenges. The main is that you have basic fragmentation grenade, which is very good at the time, but then nothing. You will find ever stronger weapons, but not grenades, with exception of perhaps plasma grenade, which indeed does more damage, but also of different type, i.e does not scale with explosives but weird science.
Aside from that, issues might be with available amount and price of grenades.