Well, that cassette issue is one of the reasons I've said a thousand times, don't electrify your favorite bike. Mainly, if a bike rides that beautiful, it might be best to keep it that way. You may still want to ride a light, sleek, wonderful bike on a beautiful weekend day.
lots of much cheaper bikes make great e bikes, such as entry level steel framed hard tail mountain bikes with 7 rear gears. Those accept the 7 speed motor wheels easy. The other option is still pretty simple, just replace the shifter on your bike for a 7 speed one, or an old school friction shifter.
Bear in mind, the way you describe wanting to ride, you will only ever use one gear, the highest one you have. You could fix that thing in high gear, and throw away your shifter. And you likely will start wanting a new crank, like one with 48 teeth on the front gear.
In short, it starts to really make sense to stop thinking convert your favorite bike, and just look for a perfect bike to convert. It may shock you to find that one of the best bikes to convert for a commuter, is a 7 speed beach cruiser. Heavy pig yes, but it will not use as much more watts than a light bike to cruise 25 mph as you think. Nice comfortable riding position, affordable, and again, you only need 7th gear anyway. You never feel its weight when you are pushing 800w into that motor. When its wet, you will love the big fenders. Ideally buy one with the rear rack welded in as part of the frame. Load up those panniers with the stuff you need to commute, and groceries on the way home.
If you really want to convert that bike, then one option that may make more sense is a more expensive mid drive.
Re waterproofing, one trick I like to use is mount that controller close to the back of the seat. All your plugs tuck under the seat, and your ass hangs out enough to keep the controller dry while you are moving. bag the seat to park in the rain.
More expensive kits are out there, with much better waterproofing than that cheap one. Starting with a better sealed controller box, and waterproof higo type plugs.