If an online brokerage account charges $25 for each transaction, wouldn't it be not worth to transact in lower amounts?

If you don mind newly approved brokerage then you should take a look at tigers. They are offering extremely low fees for SGX till end of the year. I think there might be perks too if you use a referral code

Yes of course. If you're able to invest only $500 monthly, i'd say go with fsmone rsp instead to save on fees.

Assuming the $25 fee you're talking about local brokerages on local stocks, you can use dbs vickers cash upfront which is $10 fee instead and stocks go to your CDP acc. Can sell your cdp stocks afterwards in fsmone which also charge $10 fee.

If you're starting to invest, dca is a good approach if your investment sum is small (<$10k).