You leave your known-compromised phone at home when you're going away (be that 100m, or 10,000km) to do the business you're worried about. If you're likely to need numbers from your compromised personal phone for use on the burner, carry an address book - which may not look like an address book. Say, edit the "Our list of international dealers" page in a relevant manual to include the appropriate numbers with hints. (That idea has been burned ; don't use it.) It's vulnerable to checking, but is it likely to
be checked?
Where do I get a secure phone in SE Asia?
Sorry, no idea. I applied for jobs in SE Asia, but never got assigned there. Ditto Australia and South America (well, I did some desk work for Venuzuela).
If it were Seoul, you'd probably be OK buying a phone with cash in one of the multi-story indoor markets for electronics - from "lucky cat waving" machines to that one-armed-bandit thing. Assuming they don't require your ID card number instead of/ as well as your credit card details. Otherwise ... sorry, can't help.
OH - PAYG ("pay as you go") payment updates on the burner - cash only again 9if you can, or laundered through something like an iTunes gift card (itself brought with cash. Plenty of people have let their tradecraft discipline slip for a routine thing like buying more phone credit, and one connection the spooks can draw is enough.
At least in Asia, you are unlikely to attract any attention by wearing a hygienic mask. Hat and different glasses to your normal too, of course. Jacket off or on when changing public transport vehicles. The usual.
I have proton but that only works if they have proton
Isn't that a US provider? Do you CC all your emails to NSA@gov.us ?
Can you get to a (cash-only) internet cafe ("IC")to set up, say, a half-dozen Yahoo (or some other big provider) accounts for use on less vulnerable devices next year? They'll (Yahoo, whatever) almost certainly log your IP on setting up the account (and on use, every time, too) and keep it permanently (or just send the details to the NSA) but if the IC re-uses it's security/ surveillance tapes (hard drive space) every 3 months you may be OK there. Cash for the internet and the coffee. That has caught people before.
It's a shitty situation if you don't have people you've got confidence in. I had colleagues in the trade union movement who'd been harassed and repeatedly sacked for decades before I met them, and did had learned to trust no-one a long time ago. As well as never meeting US embassy officials without 3rd party witnesses who you didn't introduce to them.