Thanks for advocating for this big guy!
Since you are a volunteer, and it sounds like the shelter currently owns him, have you discussed your ideas with staff? Is there anyone there who usually coordinates transfers? If so, I recommend working with them, making sure they know which groups you are contacting. I've been in the situation where a bunch of folks all contacted rescues they found, and the same rescues got multiple e-mails about the same dog! It makes your effort look more professional and keeps things organized if you make sure you aren't all asking the same group. Even if a group you contact can't take him, if you make a good impression they might be open to taking a different dog in the future.
Do you have any info about the reasons his past 6 adoptions haven't worked out? That might help you tailor your requests. For a totally-made-up example, if he is fear-reactive to car traffic and strangers passing an adopter's house, he might do well in a more rural setting, and being able to make that argument/explanation to the possible rescues will let them better assess whether he is a fit for them.
In addition to Great Dane rescues, it may be worth reaching out to general "giant breed" rescues. Often these are folks with more experience handling the extra-large dogs, who might be more comfortable taking on a large dog with behavior challenges. Do read their websites carefully though, as I've found many in my area will not take a dog who has bitten a human due to concerns about legal liability when managing a giant breed dog who is stronger than many humans.
You may also want to look at the listing of groups on the Best Friends network: https://bestfriends.org/our-work/supporting-network-partners All these groups have gone through a short screening process to join the network, so your odds of finding a responsible rescue are high!
#TransfersandTransport