I think the Harkles giving up the leasehold was a combination of both your suggestions, Martha. First, it would fit that it's in the Harkles plans to totally cut ties with the Royal Family and the UK, period. Maybe not Harry, definitely Meghan and Harry won't voice any objections with her.
Second, it may have been their thinking that with the move to Canada and the US and "stepping down" as working Royals that making the lame statement that Frogmore Cottage would be their UK base would keep it hanging and thus they wouldn't have to pay for any upkeep, I truly think they were convinced they wouldn't have to pay back the money for the renovations, if they kept insisting they were coming back some time to the UK.
I think they were shocked when the UK public demanded they pay back the taxpayer funds used for Frogmore Cottage's renovation. If you go back to the statement that the money had been paid back in full, it read like that of a petulant child. I also think they were taken aback about paying rent every month on the property, they expected to be let off the hook, again.
They had that expense, plus the $100,000/mon. of running Costa del Sussex plus the money spent for the luxury cars and security. Then there's Meghan's ongoing legal fees with her lawsuit and those are going "ka-ching, ka-ching" every day. The advance money from Netflix was probably spent in a week as any money from their Zoom appearances. That leaves only the estimated $2-5 million a yr. from Netflix from now on. The first thing was to unload Frogmore Cottage even though that money isn't even a drop in the bucket. They aren't going to use it.
It's a splendid home for Jack, Eugenie and the baby. I hope they don't have to do too much to undo the Sussex decorating to adapt to their tastes.