Dutch Bike Winter Cleaning
To my standard, if I kick/push the crank (freewheel direction) as hard as I can and it does not rotate more than 1 rotation after the push, I know that it is time to service. I actually aim for 1.5 to 2 rotations

Usually lubing the chain will do the job. This happens after one winter, and the chain usually shows signs of rust. This is on my Cobra steel dutch bike, with partial chain guard.

It is my winter bike. So I rarely wash it after rides on rainy winter days. A wash every fortnight/winter would suffice. The idea is that winter bike requires less maintenance in tougher weather condition. So if one have to do a lot of maintenance for a bike in one winter, maybe it's not really meant for winter.

Regarding the screws/bolts, I have replaced them all with titanium material. They do corrode, but not as bad. I also put a little lithium grease on them, and by little I mean a very thin coat. The grease never cause me any trouble.

Regarding the spokes/nipples/hub, unfortunately there is not much you can do except replacing them when they fail. I heard a rumour from our local bike club that you could grease the spokes/nipples a little and slow down the corrosion. It sounds tedious so I have never tried out.