I would be spending 2 days in the Lugano area, 1 day in Bellinzona, visiting the castle, and Madonna Del Sasso and proceeding to Lugano in the evening.
The second day I plan to spend around Lugano, walking around the town, going to Morcote and San Salvatore.
Any suggestions would be great, any smaller places I could visit, or any walks I could do, or where I get the best sunsets on the lakes, any must-have local food specialties, and any frescoes/paintings/churches which I could visit.

All you ever wanted to know about Lugano:

https://www.ricksteves.com/watch-rea...of-switzerland

I had a wonderful mushroom risotto at La Tinera.

You can do a boat ride on lovely Lake Lugano or take a postal bus into the hills where there are many bucolic walks. I did not find that much to do in Lugano itself wonderful looking city as it is.

Madonna del Sasso is up high above Locarno, on Lago Maggiore. So that's a whole different city on a whole different lake. To get to Locarno take the train from Bellinzona. You reach the church of Madonna del Sasso by a funicular based near the train station.. After you visit the church and take in its expansive view of the Alps and the lake, you can continue up by gondola to Cardada, a foresty park with a broader view, and even further up by a ski-lift to Cimetta, if open.

Also from Locarno you can take a boat to the beautiful Borremean islands and/or postal buses back into the surrounding hills.

So lots to do and not nearly enough time.

If you go to Morcote and San Salvatore on the same day, that's a pretty full day on its own.

I recommend an early morning walk in the city park, all the way through to the pedestrian bridge over the small river/creek flowing into the lake. http://www.luganoturismo.ch/en/see-d...03/parco-ciani

Also enjoyable is a walk along the lake to the city's new (almost) cultural district. Nice views, interesting museum and buildings. http://www.luganoturismo.ch/en/see-d...ugano-sede-lac

Re meals: our two favorites have consistently been Bottegone del Vino and La Cucina di Alice. http://www.lacucinadialice.ch/ (eat inside, at a lakeview table, reservations strongly recommended).

Even as someone who travels HARD, I think you are trying to fit a LOT into time spans that might not work. For example, as Mimar notes, your plan to include Madonna del Sasso on your day that includes Belinzona is, at best, ambitious.

Good luck!