What is the minimum amount to DCA with IBKR to avoid $10 activity fee?

You do not need to "maximize" your $10 minimum monthly commission to make the most out of it. Instead, look at it from a different perspective - based on your DCA strategy, will other brokers cost less than $10?

For Irish-domiciled ETFs listed on the LSE, IBKR is often the cheapest if you DCA monthly. Your other options are mainly Standard Chartered and Saxo, and they charge at least 10 USD in commissions per transactions. That doesn't even factor their foreign currency conversion spread and charges.

If you are investing purely into US markets, TD Ameritrade may be a better option because they charge no commissions, no monthly minimums and no custody fees.

IBKR's inactivity fee is more of a number of transactions rather than an amount to avoid the fee.

But assuming you're on fixed tier pricing (min USD 1 comms), that would be about 10 transactions to avoid the inactivity fee.

In terms of DCA amount, since the comms are charged at 0.005/share, to avoid the $10 commissions, you'd need ~2,000 shares transacted to waive the fee.

My personal advice is, if you're not at the USD100k yet and you're not looking to trade much, you can stay on another broker first and transfer your positions to IBKR in the future.