Tiger Brokers vs TD Ameritrade - which is better for beginner - has low capital, looking for low fees as much as possible, user-friendly interface?

TD Ameritrade has the lowest fees (since they don't charge any commissions) and have close to spot FX. That being said, TD has ridiculous account approval times (4 months and counting). So, you might wish to use Tiger instead since it comes pretty close in cost anyways and has more market access compared to TD.

I'll try to answer your questions in the order that you asked them:

1. Syfe Equity 100 is pretty well diversified. However, you can use specific ETFs to overweight on certain sectors (e.g. tech (XLK)) if you wish to by adding some tech ETFs in your DIY portfolio. Additionally, it is also possible to add certain smaller sectors that are not already covered (e.g. weed stocks (MSOS), semiconductors (SMH), green energy (ICLN) perhaps).

2. You can, EMQQ is one that focuses on emerging market tech. VWO is decent as well.

3. Several famous investors (Ray Dalio, for one) advocate for a small allocation of gold in one's portfolio as hedge. Personally, I allocate about 2-3% of my portfolio to BTC (which hedges against falling USD). Do note however, that in March, both Gold and BTC fell quite a bit and when the market bounced back so did gold and BTC (meaning that they currently share a correlation with stocks).

4. You can consider money market funds such as Syfe Cash+ or Endowus Cash Smart. Alternatively, other insurance savings plans e.g. Gigantiq are also viable (I think they're still open for registration, last I checked).

Hope this helps!