You have established that you are a tourist by returning home after each visit. That's fine. If you hold an appropriate passport, you could come without a visa and be stamped in for 60 days. You can then extend this for a further 30 days at the Immigration Office where you stay
1500 years ago is irrelevant to the sin sod issue and it's context is not religious at all. Today just 0.1% of the Thai population are Hindus. You need to do some more reading, this is not a subtle nuance like Protestants and Catholics
Each family will be different, and their approach will be influenced by the bride. Have many long conversations with your lady about what she and you want, and agree it clearly before engaging in a negotiation with the parents!