There are many 50+ on DTVs Inc those moving from retirement visas. There are people in the 70s on the DTV doing Muay Thai. So you can get a DTV and apply yourself online via your local Thai Embassy. You need to decide which route you will take and provide proof of activity as you detail in 4.You should also check the main DTV group on FB for lots of tips in applying.
It is ok to rent direct from an owner they just provide letter detailing your stay. You can submit your savings account statement and the account statements which your salary is paid into