If an attacker has the first 6 words (in the correct order) of my 12 word Trezor backup seedphrase, how long would it take the attacker to brute force crack the remaining 6 words?

Is it necessary for me to wipe the device and create a new seedphrase?

Based on the numbers achieved by this guy, you could do it in 57 thousand years.

This scales with the number of GPUs you’re throwing at the problem: get 10 000 times as much (seems achievable today) and you can be done in less than six years.

A powerful quantum computer, the kind that could maybe exist in six years, could do it in two.

That buys you more than enough time to move your funds to a different seed, which you should absolutely do regardless.