Life in the UK for Harry and Meghan will be similar to life in the US for them, i.e. a flop in terms of "making it". They will be more annoying, however, because Harry will keep trying to make himself and his kids working royals (i.e. representatives of the King) - but that won't happen.
Some people/institutions/charities will be a little confused as to how to approach him because he is still the King's son. Some might be willing to work with him (less so her) because he's "important", but he won't be of much benefit to them.
He will milk Invictus and the othe veterans charities he's involved with, because that's all he's got. I don't think either he or Meghan are going to get many new roles, however, and any organisation that associates with him will appear a little tawdry, because he's a lower-tier royal with no official royal recognition.
On a private family level, it's up to Charles, William, etc. what type of private relationship they want with him, but Harry, Meghan and the kids will never be part of "official" royal family work.
It's easier for Charles to act the role of granddad every now and then than it is for William to perform the role of uncle, but, while William can mostly ignore Harry, it will be less easy for him to ignore his nephew and niece, especially as he becomes King, and as George eventually becomes King. That's for them to work out privately, though.