At the risk of being pedantic, what you got was a visa exemption, not a visa, but yes you can extend it 1x 30 days for 1900 baht.

You can extend the initial 60-day entry 30 days at any immigration office for 1900 baht. Then do a border hop for another 60-day entry and repeat. You can even do a border hop on the day before the visa expires and get another 60+30, which means you get nearly 9 months out of the 6-month METV.

The one opposite Ranong is Kawthong (a distortion of "Koh Song") and yes you can do a visa-exempt border hop there (max. 2x per year land/water hops).

No it is not compulsory to do an extension. Border hop with METV at Mae Sai will result in another 60-day entry. Compare w/extension 30 days 1900 baht vs. Mae Sai run from CM by bus (320-638 baht, depending on class) + 500b for the Myanmar border run). So border run is a bit cheaper, but takes a whole day, +60 vs. 30 days.

If you just need a 30-day border hop, Ranong is probably easier, maybe even a direct bus. Still a 5-6 hour ride so perhaps do an overnighter.

Technically-speaking it's a bilateral agreement, not a visa exemption, but pretty sure it can be extended same same 30 days (at the discretion of immigration officer, as usual).

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'Kay, you win ;-)

Cos I've been going in and out for about 20 years and only once been asked to show money. But lately there are numerous reports of it being enforced.

Nothing new, except that up until quite recently it was rarely enforced.

Oh so you've already done 2 tourist visas and 2 visa exemptions in the past year? There is never a guarantee that you will get another. Bring proof of 20k Baht (preferably in cash) also for the return to Thailand as border officials are reportedly requiring this even if you have a tourist visa. As for other documents in Penang, just 2 photos and your passport with a couple clean pages + signed photocopies of your main page and Malaysian entry stamp page. And it helps to dress politely ;-)