What kind of weather can I expect in Scotland in the Spring?

As you asked for a "a human description" I have kept my answer anecdotal without reference to stats that you probably have already read anyway. I lived in Glasgow for 28 years so know the weather personally. Though bear in mind that the west coast gets significantly wetter weather than the east. The West Highlands get significantly more rain than the West lowlands.

As you experienced in your holiday before, Scotland's weather is quite erratic. Spring is no exception to this. Any season can have good or bad days. I've seen a days with the temp above 22C in February and I have seen it snow and hail in summer so any statistic that says the average is something something should be taken with a big pinch of salt. Average weather is not the dominant norm in Scotland. You get a wide spread to either side of the average.

Spring usually sees a period of heavy prolonged rain at the tail end of winter then heavy showery weather before moving into summer. The weather you described of 15 mins rain and 30 mins sun sounds more like spring than summer to me. Summer usually has rain at night but dry in the day. So if you like showery then you could be OK in April.

The exact timing of the weather described above can be problematic. Winters can last a little longer and summer can come early. I have seen it rain constantly from March to May without a single days break. I have also seen the converse. I saw decent snow fall twice at Easter but also seen it warm enough to go outside without a jacket.

I think what I am saying is that nobody really knows what the weather will be like in April in Scotland. Anything is possible. You could get lovely weather all through your holiday but you could also get gale force winds and storms too. This is one of the joys of visiting Scotland and will be sure to surprise you every time you come.

One advice I would close on: the weather is often localised. Jumping in the car and driving 50 miles north or south can completely change your weather. I have seen it sunny at Crianlarich but blizzards at Bridge of Orchy. It can be pelting cats and dogs in Paisley but bone dry in north Glasgow. If you keep your plans flexible and have easy transport, then you can often dodge the worst of the weather.