Firstly it is easy enough for you to do yourself. Agents just cost you money. The only thing you need to prove is that you can support you and your wife with your income in the UK. You can have zero in the bank account but you will need to show last 6 months accounts from memory.
Be careful with the word silly reasons. A rejection is a rejection.
Long term visa first ( you can use proof of assets
From your own bank) then agreements for permanent / rented premises and then go to the bank. This way no agent fees or the bank con if selling insurance
12 months as need to show consistency of earning. Money has to go into Thai baht to prove you can support yourself.
For you I would go for the deposit. I understand you can earn interest now. You can withdraw during the year but put back 3 months before renewal. Easiest way is to leave in and place in an interest earning account