Is buying Singapore Savings Bonds monthly through DBS savings account counted as an investment transaction?

If you are talking about the investment part for DBS multiplier, the interest you get from SSBs will be counted. You can do the SSBs ladder. SSBs pay interest every 6 months. Example if you buy SSBs once every month for 6 months like from January to June, you will receive interests every month afterwards for 10 years. Every month the interest you received will be considered investment if DBS dont change anything. You would need $3,000 to do this as the minimum is $500 for SSBs.

Any of your stocks that pay dividends will also count towards the investment tier

If you buy stocks using DBS vickers that will count towards the investment tier

You can also take a look at DBS website they have promos for investment into etf and trades