Now if you say that you know only 6 words out of 12 then I would say it's near to impossible brute force and crack the rest of words because there are lot of possible combinations as you are missing 6 words out of it.See this
answer where only one word is missing and we have so many possibilities so you are wrong on this part I would say.
There are many posts on the forum that explain how demanding (or rather expensive and time-consuming) everything that exceeds 4 words is - because @o_e_l_e_o explained it in one of his posts on the example of seed where someone needed about 30 hours for 4 words with note that he did not do it with his computer. Only 5 words would extend that same procedure to even 7 years.
This person bruteforced 4 words:
https://medium.com/@johncantrell97/how-i-checked-over-1-trillion-mnemonics-in-30-hours-to-win-a-bitcoin-635fe051a752On his own computer it would have taken him 25 years. By spending $350 renting cloud computing he was able to do it in 30 hours. If you make that 5 words, rather than 4, then it would have cost him $350*2048 = $716,800 and would have taken him 30 hours*2048 = ~7 years.
I think 4 words is the limit of what could be called "easily" brute forced (and even then, it's not
that easy). 5 words is potentially possibly, but only with a significant amount of time, money, and resources.
Also from the same thread, one interesting fact when it comes to 12 word seeds.
Because there are this many possible valid 12 word seeds:
340,282,366,920,938,463,463,374,607,431,768,211,456
And there only approximately 30 million bitcoin addresses with balance on them. Divide those two numbers and you get a roughly 1 in 11 million trillion trillion chance of finding a collision. (Now, this is not quite accurate since any seed can generate potentially billions of addresses, but you get the idea.)