We did this trip in early August 2019 and really enjoyed it. We also added Viking's Geneva and Zermatt pre-cruise extension on the front end. That was a wonderful 4 days.
Unfortunately, Viking doesn't always use the most centrally located hotels. On our trip, it was the Renaissance Tower Hotel in Zurich and Le Meridien Etoile in Paris, though they sometimes use other hotels as well. Both hotels were in more of a business/convention area rather than a central tourist location. The good news is that public transportation in both areas was easy to use. If not comfortable with that, taxis are always an option. There were several good restaurants within easy walking distance of both hotels. Though we would have liked a more central location, it really didn't impact us in a negative way.
I don't see lunch and dinner on your own as a negative. It's a great opportunity to sample local cuisine, which people on cruises sometimes neglect to do. Though Paris and Zurich are expensive, there are restaurants and markets within a wide range of price options.
Weather will be chilly at night and probably in the 50s and low 60s during the day. Generally, light jacket and sweater weather, especially if you're used to Chicago weather. To me, the big benefit of doing this intinerary in April is you may encounter fewer tourist crowds.
As I recall, you are offered at least one included Viking tour in both Paris and Zurich so you need to factor that into the equation when assessing value. At the end of these tours, you don't need to take their coach back to the hotel but can stick around in the tourist areas then make your own way back to the hotel at your leisure. There will also be several paid tour options offered by Viking if you choose to partake. The Viking on-site host can also be helpful with transportation and restaurant recommendations.
I wouldn't hesitate to go!