I think that it will play out very much as the Duke of Windsor and Wallis. Not an original observation, I admit, and I don't mean the 2 situations are exactly alike, but there are a lot of parallels.
Not saying Meghan is Wallis. But she was never going to submit to the constraints of being in the RF. Harry seems dim in the way the D of Windsor was - not least because he failed to put her in the picture as to what marrying him would mean.
The parallel, I think, is that Harry will miss his role and find it difficult to carve a place for himself outside the UK. What are his skills? He has turned his back on the job he was 'trained' to do. I guess he thought 'security' came free of charge and is now finding just how incredibly expensive that is.
Also, there is a danger they will become inward-looking, like the Duke and Duchess of Windsor did, and get bored with each other. I suspect Meghan will tire of Harry first, though. Once out of the RF, he's a bit lost.
I don't wish this on them, before anyone jumps on me. I hope they will be happy and find a rapprochement with all their relations, Meghan's included. But to do that they need to quit the telling tales out of school and washing dirty linen in public.