Could I reduce my tax liability by sending my pension money from my bank in Austria to Thailand in separate transfers, such as 900,000 THB to my account and 300,000 THB to my wife’s account, and filing separate tax returns?

If the source of the funds is your personal pension, then by remitting the funds to your wife’s Thai account, it doesn’t cause a tax implication for your wife, but still for you who remitted the funds.

Your tax return would need to include both the transfers to your Thai account and to your wife’s Thai account. You cannot simply pass the 300k on to your wife’s personal income tax.