Is this considered rebalancing of my investment portfolio?

Not really. When it comes to rebalancing, you will somewhat be selling something to get something else to balance your portfolio.

For example, you started investing with 50% stocks and 50% bonds. Over the years, the stock market performs really well and you now have a portfolio not 70% stocks and 30% bonds. You can rebalance by selling 20% stocks to buy 20% bonds so that your portfolio get back to 50% stocks and 50% bonds again.

Your current position looks more like building up your portfolio by the way of diversification. You feel you are too heavily focus on Singapore market and wants to diversify your risk by starting to buy global ETF.

If you are rebalancing between local and global exposure then YES. Eventually you will get to the point % that you want for your portfolio.

Not quite, but in a way, diversification will give your portfolio a form of rebalancing as you introduce new asset classes/geography/sectors, etc into your portfolio. To me, strictly speaking, only if you had a portfolio that you didn't wish to increase the size, then selling off some current holdings and buy more of other holdings in the portfolio is diversification.

As an example, when I bought my first share years ago, my portfolio wasn't diversified. But as I added other shares in other sectors, as well as UTs, my portfolio was eventually more spread out. This is quite similar to what you intend to do.

Yes of course.

with STI only you have 'home bias', Singapore is small economy from a global perspective.

Rebalancing as a risk reducer is often recommended, I am contrarian, 'let the winners run', and may only add new funds with your initially defined asset allocation composition.

Excellent that you are now interested in more global ETFs, VT and VOO would be obvious choices for stock ETFs.

For Singapore S-REIT ETFs could be more attractive than STI, particularly the Lion-Phillips one.