As a tax resident of Thailand, how are my investments with interactive brokers (personal account) treated for tax purposes? Am I only required to pay tax on the funds that I transfer into Thailand, and are there other tax implications to be aware of?

This is a general overseas investment account. You will be potentially taxed on funds that are remitted to Thailand. You are taxed on the capital gains. If you do not remit the funds to Thailand, they are not liable for tax in Thailand and do not need to be declared. If you do remit the funds to Thailand,and there have been capital gains, this needs to be declared on your tax return.