What country in SEA is the easiest for legally renting and driving

Nobody cares about the law. Local insurances might not work properly, so don't crash into others.
Having a international drivers license is recommended, however they usually don't care about a motorcycle license.
Try 'motorcycle rent [city]' on google to have a good impression.
Not everything is online.
Longterm rent is cheaper.
Cops in Bali like to stop foreigners to get bribes, but they don't do that in Java.
Usually you shouldn't be bothered by police but check it out online, some areas in S-Vietnam might be tricky in terms of cops.
You can rent everywhere some kind of chinese motorcycle, getting a proper bike might be difficult.

Crossing boarders with bikes can be difficult, get information ahead.
I would recommend Laos, North Vietnam, Sulawesi, Sumatra, maybe Borneo, maybe Myanmar(more banned areas due to fighting than expected, not everything is properly listed online).
Go to all the national parks, driving there can be fantastic and they will be your highlights.

Java and Bali are not bad either, I would recommend Sumatra, Borneo(Malaysia and Indonesia) and Sulawesi however instead.

Always get a local sim card with internet, it's worth it and like 1$/card, 1$/GB, sometimes LTE.

You can buy helmets there BUT GET PROPER RIDING GEAR AT HOME!

Don't ride more than 5min with sandals and swimming shorts, at one point one might fall and even superficial scratches tend to heal worse than at home in my opinion.
One accident might finish the trip.

Seems very unsafe. I would personally not do it.