There’s always a balance to be struck, and good enough is good enough.
Neverthless, although I used the term power to weight ratio in my post, which is more normally used when discussing racers and competition riding, it is still relevant to tourers and ordinary people. If a bike is heavy and not very nice to ride, that is likely to spoil the experience.
The fact that as things stand the bike is only likely to be ridden four times a year, might itself be a reason to spend more, because if it’s nicer to ride then she might want to use it more.
In contrast, DrP, having read some of your posts (e.g. South Downs Way in a day on a single speed), I suspect that spending a similar amount of money on an upgrade for one of your bikes might not make anywhere as big a difference to you: you are very fit, very motivated and would get out and ride regardless.