How should I invest using IBKR if I want to minimize fees especially the monthly activity fee?

I will assume that you are investing into Irish-domiciled ETFs on LSE, because that is what IBKR is best for. For US markets, there's TD Ameritrade and Tiger Brokers.

You should be fine since you are investing $1500 a month. With that investment amount and interval, there is no other broker that can match IBKR's commissions, even with its 10 USD minimum monthly commission. Standard Chartered charges 10 USD minimum per transaction and doesn't offer the excellent forex rates that IBKR has.

Just consistently invest, and once you hit 100k USD AUM with IBKR, your commissions will become even lower.

Sadly, there is no other way to go about doing this other than fulfilling the US$100,000 account value. IB has very competitive fees for most areas, just that the monthly activity fee is a obstacle for most retail investor who is just starting to overcome.

I have heard from others that they consolidate their investment amount with friends/relatives to cross the US$100,000 account value, of course with detailed management of each and individual investment and how to operate together efficiently. So perhaps, this is one of the alternative you can consider.