You said you have booked long term. Do you have a contract with the owner? If so you can lodge the TM30 application yourself and do not need the owner to be involved at all.
Keep using 30 days until they stop you. Otherwise no limit. It is more of a problem if you do it back to back without any breaks in the middle. Hence why Oil and Gas workers get away with it on a 28 and 28 or similar.
Correct 8k AUD or equivalent. Not really ridiculous for staying 6 months or more in Thailand. Works out as 33k a month. If you have the money in the bank it is no big deal. But you also need flight tickets and accommodation details as well.
People say I am crazy when I say this, but never go to a female IO on arrival to Thailand. The likelihood of them stopping you is much higher than a male officer in my opinion and experience. But as mentioned above, not really worth chancing it all the time, especially when there is no valid reason for you not to get a visa. So quite rightly it looks suspicious to immigration, especially if they are back to back or very close together.