You can send 2 million THB with WISE in a single transaction to certain (3) Thai banks, that is a WISE limit.
Other banks have incoming limits (Thai regulations), it's all on the WISE FAQs.
You have to give a reason for the transfer on the WISE site.
I have a Bangkok Bank account and sent 8 million to myself from the UK in one day, they did not contact me for any kind of declaration and I moved the money out to my wife without issue.
It is not cheaper to split into smaller transactions with WISE.
Fees for funding the GBP amount to WISE vary by method, Debit Card, Bank Transfer etc
The WISE fees may seem large but as they use the mid-market rate it was still cheaper than any other method and I believe it is still so.
This information is specific to the OP's requirements, GBP to THB, fees with with other currencies are different and there may be cheaper alternatives.