What is the most cost-effective way to DCA into IWDA every month?

Well, IWDA is an ETF traded on London, so question reduces down to what is the cheapest way of buying stocks there - perhaps you can look into Interactive Brokers for a very cheap solution that scales well at larger sizes.

In terms of the amount you need every month, the trade cost is minimal so the question is more how much you can/want to invest each month. As mentioned in comments though, there is an Account Maintenance Fee of USD 10 a month, which may bite if your account is just starting out (its gets waived once you hit USD100k (!)).

When you are starting out or at smaller trade sizes, you can look into Saxo Markets to start off with, they run off percentage fees (as opposed to fixed fees like IB) so will be more economical at smaller sizes but once your trades/portfolio gets larger then it makes sense to switch over.

Have you considered other options other than IWDA?