“at what point would you recommend changing working parts ?”
There’s two ways of going about it.
If you work on the assumption that the bike is used regularly and serviced every year then would leaving a worn chain on “because it works” last another twelve months without issue.?
The second approach is to ride it until it shifts like a dog and or breaks.
The whole point of maintenance is so your pride and joy doesn’t let you down half way through a ride, or at the very least eliminate the chances as much as possible.