Not sure as some details are missing.

You can apply for a Single Entry Tourist Visa which gives you 60 days of stay, than go to Malaysia as planned, come back into Thailand without a visa and get 30 days Visa Exempt Entry, which can be extended once with another 30 days.

This will give you enough time in Thailand.

Okay, This picture of Immigrations would explain it in my point of view:

AS you are not allowed to be longer than 90 days inside the country, you normally do not have to make 90 days reports.

BUT some Immigration Officers start telling the foreigners to make 90 days reports as they seem to be very long times inside Thailand based on the fact that they are unable to travel.

With not knowing where you are, the only option you have is to visit your local Immigration Office and ask them if they want you to make the reports and when you have to do this.

@Chelsea *****

No, you apply separate for the Multiple Re-Entry Permit, which cost 3.800 thb.

Correct, if you arrive in Thailand you pick up the work permit and after 60 days you apply for a 1 year Extension of Stay based on employment.

When that Extension is stamped in your passport you can apply directly for the multiple Re-entry permit to travel during that year.

You are in the UK, filled that in and did choose employment like this picture

@Martin *******

Than I guess you never entered Thailand using the Non Immigrant O-A visa.

Okay, lets get this clear:

For the conversion into the Non Immigrant O visa there is NO seasoning of the money required.

For application of the 1 year Extension of Stay the money must be at least 2 months in a Thai bank account.

As the are named the same, retirement visa, the conversion into a visa and later an application for 1 year Extension of Stay are two total different ways of stay with different rules, regulations and requirements.

1. I guess as long as you have a bank statement that the complete sum of 800.000 Thb is deposit from abroad I do not see the problem.

2. Yes, the foreign bank account is not mentioned at Immigration.

3. Yes, as the money must come from abroad, there is no mention of specific country.

To do it legally: You need the Non Immigrant B visa and a Work Permit.