There are a lot of things to consider before you decide to make the move.
Have you visited Oban before? If not, I would go at different times of year to experience what it is like, especially Winter. Weather wise, it is really not that different to North Yorkshire, but it can appear to be a very bleak place in the winter when there are very few tourists and a lot of places close up as a result.
Oban is a different town in the summer than it is in the winter - population wise, there are approx 8,000 residents, but the number triples in the summer due to the amount of tourists, this even impacts the phone signal of the major phone providers.
To address your concerns about darkness, sunrise is around 1/2 hour later than North Yorkshire but sunset is at the same time.
Why is your husband considering Oban? I would imagine that all the things you have mentioned about a rural lifestyle can also be experienced in Yorkshire, so what’s the draw to Oban in particular? What does Oban offer that you can’t get where you are currently?
There are not may suitable rivers for kayaking and for fishing, permits are mainly required, so I would say Argyll is a strange choice for these reasons.
Oban and surrounding areas are beautiful and would be a great place to grow up, but the accommodation is limited due to the sheer number of air bnb’s in the area. I would definitely look into the job and housing market to see what would be feasible for you.
It depends what you think is isolated - it is a pretty big town and facilities are expanding all the time. There are some really good primary schools but only one high school that covers a wide catchment. Fort William is under an hour away (nearest Home Bargains and McDonalds) and Glasgow is approx 2.5 hours by car or 3 hours by bus or train, which both serve at least 3 times per day. Do you both drive? A car is essential.
The hospital is small and although has an A&E, mainly deal with minor things; any major surgeries / complications are dealt with in Glasgow. There are a number of dentists which take nhs patients and there’s a big gp surgery in Oban and another in Connel, 5 miles away.
It is difficult to make the move to any new place without support of friends or family, but as you have a baby, attending parent and baby groups will help to get to know other people, so will make it a bit easier.
Drugs have been an issue historically, but no more than other similar towns - the police and council are working hard to address the issues.
https://www.argyll-bute.gov.uk/moderngov/documents/s172461/20210302%20-%20DOC%20-%20AB%20Qtr%203%20Update.pdf
Lots to consider and weigh up the pros and cons when doing more research.
Happy to answer any more questions you may have.