Help Deciding on a "Gish" Class

I have been invited recently to a level 20 oneshot/twoshot and and am struggling to decide on the class I want to play so I thought I'd consult the council.

For context the setting is volcano with an expectation of fire resistant/immune enemies and the other three party members are a Metal Kineticist, Pistolero Gunslinger, and a Life Cleric. We start at level 20 and have free archetype as well.

I am personally a big fan of "Gish" classes which I put in quotes because what I really like is the hypothetical versatility that comes from buff spells and adding cool magical damage effects/riders to my weapon strikes. My current dilemma is deciding between Magus and Inventor (I know it isn't a true Gish but it fulfills my personal criteria).

Magus seems really explosive and has spells but also feels like they'd run out of steam fast and the manipulate trait on a core action is spooky (although I'm most interested in Twisting Tree so that does alleviate some of that). Also the upcoming Faydaan/Maelstrom Magus is tempting so idk if I wanna run Magus twice.

Inventor seems like it's got a lot more consistent damage with flat modifiers/dice and very cool weapon modification stuff but also the fact that you can fail your actions and damage yourself feels really rough.

Id really appreciate people's opinions especially if they have experience with these classes at high level or high level play in general. Also please suggest any alternatives you might have in mind, it's a big system so I'm sure there's more out there I haven't even considered.