Staying at a small hotel at the Navigli

Note: I have been to Milan, but only briefly, and have not been to the Navigli District.

I believe that hotel is about 1/3 mile from the nearest Metro station, which seems to be Porta Genova. That station is in the opposite direction from the center of town. How do you feel about walking that distance twice a day just to get to the Metro? I find subway systems much easier to use than buses; there's no doubt where the stations are, and they'll be marked on just about any map you get hold of. The ticket-vending machines will probably be easy to use; in some cities there's an English-language option, though I don't know whether that's the case in Milan. But subway stations are often a lot farther apart than bus stops, so an unwillingness to use buses can mean a lot more walking.

I'm assume there are bus stops much more convenient to the hotel than the nearest Metro station. The desk staff at your hotel would be able to tell you the location of the nearest bus stop for traveling toward the center of town and where you can buy tickets. Also ask about whether the tickets need to be validated (time-stamped in a machine) as you get on the bus. That is a common requirement in Italy.

The Navigli District has recently become very popular for dining and entertainment. From what I've read I suspect there will be lots of young folks out and about in the evening, especially on weekends. I would check hotel reviews carefully, looking for noise complaints. It's certainly not a deserted part of town, and I'm sure you'd be able to get taxis back and forth to the sights you want to see. What that will cost you, I do not know. If a taxi gets caught up in heavy traffic, that could run up the meter.

I'm 69 myself (and female). I have no safety concerns in Europe, but from the standpoint of convenience I prefer to stay in central locations. However, that nearly always costs more. The trade-off between cost and location is something I face almost everywhere I go. I'm willing to stay in much simpler accommodations if I'm within easy walking distance of a lot of the sights I want to see. The Borella looks a lot nicer than nearly all the places I stay, but I'm well aware most people on the forum have considerably higher standards than I do.

In a large city like Milan there's usually not one location that works really well for all my sightseeing plans, so I end up doing a good bit of walking no matter where I stay. I'd have no problem at all with a 1/3-mile walk to the Metro. Not having researched lodgings in Milan yet, I have no idea whether there are areas of comparable cost that I'd consider more convenient. In any case, your list of sightseeing targets and mine probably don't have a lot in common.

Edited to add: Milan is a business city. I believe in most cases hotel rates are lower on weekends than during the week. If your stay does not include Friday-Sunday nights and you have some scheduling flexibility, you might try adjusting your dates of stay to cover the weekend. If you're hitting something like Fashion Week, though, it may not matter what nights you're staying.

The tram at Porta Ticinese, line 3, will take you to the heart of town in no time, and the immediate area of the hotel is pleasant and very lively with locals. Perhaps one of my favorite parts of town! If you don't like noise, you will want to avoid a canal-facing room.

Question for you, though: what do you intend to do for three full days in Milan?

I recently posted a link to an article about the Navigli area - you might enjoy it:
https://community.ricksteves.com/travel-forum/books-movies/fascinating-article-about-the-past-and-possible-future-of-milan

We’ve stayed in Milan for a considerable amount of time and like the Navigli area. The hotel looks lovely. We learned to walk to the main tram line, a 5 minute walk, and take the tram back and forth to the city center, a 15 minute ride. We actually stayed further out in a private residence and used the tram system every day. Once we were within the center, we walked everywhere. There is a lot to do and explore in Milan.

Walk 5 min along the naviglio (canal) to Porta Ticinese then take the tramway (trolley) no. 3 and you will be near the Duomo in 10 min.

So, it looks like you’ve picked a really fun and interesting part of Milan to stay in, and that there are fine public transportation options. I found this site that explains how to use them: https://italyexplained.com/getting-around-in-milan/
I hope this is reassuring.