I wonder really how he thought he/they thought they they were going to manage financially or if they even gave it any thought?
There is information on this point in the book 'Revenge' by Tom Bower (it's a worthwhile read for anyone who's interested in M&H and interestingly they never sued or commented on its contents and TB said he only put in what he could independently verify from I think three sources).
Anyway, when M moved to London to marry H she obviously continued to keep in touch with US friends and contacts, and regularly moaned that as a member of the RF she couldn't accept any of the gorgeous freebies she was being sent and had to send them back. She also wasn't allowed to do deals with fashion houses, handbag and shoe companies, etc and get kickbacks for all the sales she was generating by wearing their stuff. As a former influencer, this kind of thing was her bread and butter before, so along with Harry's tiny cottage and disappointing bank balance, this was all contributing to madam's dissatisfaction with life as a member of the British RF.
Said friends and contacts apparently told her repeatedly that she/they should just move to the US, where they'd be in HUGE demand and be able to monetise everything and get stinking rich. And that happened initially - they got unprecedented deals with Netflix, Spotify and Penguin Random House to produce films, cartoons, documentaries, podcasts, books - incredible for two people who had NO PREVIOUS EXPERIENCE of producing any of those things. So the friends were right and they must have thought initially that they were set for life and were going make a huge success out of leaving the RF. But then everyone realised what a talentless pair of grifters they are. They've still made multiple millions of dollars though - estimates are that Harry made $27 million from his book alone.