I am in a similar situation (US citizen resident in Germany, have lived in Belgium for a long time, minimal ties to US).
If you want to invest in ETFs as a US citizen, you have the following options:
Invest with a US broker after obtaining professional investor status. This exempts you from EU KID regulations. However, obtaining professional investor status is difficult and requires 2 of the following 3 conditions: a) investment portfolio over 500 000 EUR b) 10 stock transactions per quarter for the last 4 quarters c) 1 year of professional experience in the financial sector making trades. This is likely the best option if you're eligible.
Invest with a US broker using a false US address. This is possibly fraud/breach of contract with the broker, but many people report that this works. The penalty for discovery is typically a ban on any further deposits in ETFs, but still being allowed to sell existing positions. I am also concerned that claiming US residence to the broker while claiming EU residence to the IRS could constitute tax fraud.
Invest into an EU ETF. This exposes you to the arcane and highly punitive PFIC rules. Generally speaking, each PFIC you hold a share in will require you to submit Form 8621 with your tax return. This form is extremely onerous and tax preparers typically charge about 200 USD to complete it. In addition, PFICs are subject to complicated taxation rules that all but guarantee your gains will be taxed at the highest income tax rate (currently 37.5%) in addition to tax in Belgium. You can use a reporting technique called "mark to market" to instead have your unrealized gains taxed as ordinary income, but this carries additional reporting burdens.
Further notes about PFICs: The definition of a PFIC is very broad and probably includes holdings such as Investor AB. Also, funds holding shares in other funds potentially creates an additional reporting requirement for indirectly held PFICs. Some PFICs complying with US reporting requirements allow you to use the QEF (qualified electing fund) election, which allows the PFIC to be taxed at the normal US capital gains rate. However, the rules around QEFs are extremely complex and I am not aware of any QEFs in the EU anyway.
If I understand the rules correctly, corporations incorporated in the US cannot be PFICs, even if they primarily hold foreign capital.
- The "options trick": you register an account with a US broker, declaring that you're a Belgian resident. With Charles Schwab, you can easily do this if you deposit at least $25000. Then, you sell a put option on the ETF you're interested in and exercise the option. See here for an explanation. Since options only come in multiples of 100, this requires you to have at least 100x the price of the ETF available in cash. The tax implications of this technique are left as an exercise.
On the other hand, if you buy stock in a holding company that trades on a US exchange and is incorporated in the US (otherwise it's a PFIC) such as Berkshire Hathaway, it is fully legal in both the US and EU, and you pay only the US capital gains tax at the normal rates when you sell (and the Belgian transaction tax when you buy). In particular, since US capital gains tax only kicks in at $41676, this is a great option if your capital is small.
You also pay costs related to currency conversion from EUR to USD and back. Furthermore, in Belgium capital gains are only tax-exempt if they aren't "speculative", which is not clearly defined.
In summary, buying Berkshire Hathaway through Interactive Brokers is one of the better options for American retail investors resident in the EU, other than being rich enough to get the professional investor exemption or moving to the US.