You can’t use the monthly 65,000 baht transfer method for your first yearly extension as you haven’t been in the country long enough to satisfy that option. You have either to meet the 800k baht into your Thai bank account option (your name only) OR get an affidavit from the Finnish embassy (if they actually agree to provide it) showing that you meet the monthly income option in Finland.
Yes because lots of transactions are sometimes missed off a bank book. Its not a reliable indicator that balances have been maintained throughout the year.