My employer offers both a pre-tax 401k and after-tax Roth 401k (but no after-tax contributions), with a matching contribution of up to 2%. I can contribute a maximum of $19,000 for 2019 to either or both accounts. For now, I plan to retire in Singapore. How should the contributions be allocated?
I'd go with the Roth 401(k), for two reasons. One reason is that I think the tax outcome will be better in the circumstances you describe. That's a guess, but it's a reasonable one. Second, since you seem to be inclined to max out that $19K, the Roth 401(k) variant works better. Pushing in $19K after tax is a bigger effective contribution than $19K pre-tax, so you win.
For health plans, 1 option is a high-deductible plan ($1,500) with a health savings account to which my employer will contribute $500. Given the tax savings, is this a good idea? The other plans are: a high-deductible ($1,000) with a health reimbursement account funded by the employer, and a low-deductible ($300) with no employer contributions.
If they're otherwise identical, and if you're good or better health, the first plan is likely to work out best for you. The health reimbursement account in the second plan is probably a Flexible Spending Account (FSA), and FSAs are not carried over year to year. The HSA can be, and if you never spend it (or only partially spend it) then it becomes another pool of retirement savings.
For long-term investing, should I open an IRA? My annual salary will be above the Roth IRA limit, so I might have to use a backdoor Roth IRA if that's better than a traditional IRA. Or am I better off just paying the additional tax and directly buy IWDA+EIMI through IB, which I am currently doing in Singapore?
Yes, absolutely, I would add to your 401(k) with the Roth IRA, backdoor'ed if need be. That'll get you up to a cool $25K per year of U.S. tax advantaged retirement savings. If you're going to be saving that much or more for retirement anyway, taking the tax benefit is a no brainer. My current favorite is Fidelity's twin FZROX/FZILX mutual funds in some reasonable ratio -- 50-50 is probably correct. Although I like Schwab for their lovely Visa ATM card too, so you might want to grab that deal at some point.
The only "gotcha" is if you end up retiring in a country that doesn't treat Roth 401(k)s and Roth IRAs well from a tax point of view -- where the U.S. tax advantages are "lost" on you. The Roth is a future bet, that the tax code in your retirement country will be, in your retirement years, kind to appreciated assets that are not (except before appreciation) U.S. taxed. If you want to hedge your bets then I'd still take the Roth IRA deal (since that's the best you can do for that leg) but split the 401(k) up into some Traditional 401(k) and some Roth 401(k). For example, if you want to split it right down the middle, you could do this:
Traditional 401(k): 65% of $19,000 (~$12,500)
Roth 401(k): 35% of $19,000 (~$6,500)
Roth IRA: $6,000 (via backdoor if required)
But the disadvantage is you'll effectively reduce your retirement contribution, since the Traditional 401(k) is pre-tax then taxed (at future U.S. ordinary income tax rates) upon withdrawal. (Roth is better if you're pegging at the annual max.) I think the future U.S. tax filings are also more complicated that way.
I don't think I'd do that. I think I'd take the straight Roth bet. In the unlikely event you end up retiring in a Roth hostile country, there are some potentially legal "tricks" you can play, such as withdrawing your Roth funds after age 59 1/2 (qualified withdrawal) but before immigrating into that retirement country.
Note that your tax advantaged accounts should properly hold the most aggressive parts of your portfolio, at least during your accumulation phase, in order to maximize the long-term value of the tax benefit. Leave any bond investing outside your retirement accounts, and if you're in a high tax bracket (sounds like it) you might consider a low cost municipal bond fund for that portion. There's no state income tax in Washington State, so you can choose any non-state specific municipal bond fund. Just be aware that U.S. munis are U.S. dollar denominated, and you'll probably want to augment them with voluntary CPF contributions, notably. (CPF assets are U.S. taxable and U.S. reportable, I'm afraid.)
If you can clock 10 years in the U.S. Social Security system, awesome. That'll vest you in another source of retirement income, assuming the rules don't change. Just be aware of crossing the threshold for long-term residence for purposes of tax expatriation. I'm not saying you shouldn't cross that threshold -- maybe you end up staying in the United States for the rest of your life -- but just be aware of it.
I've mentioned before that it's a good idea to get into a U.S. tax friendly posture strictly before stepping foot in the United States. If you'd like me to elaborate on that, let me know.
Good luck!