Yeah these are all really good considerations .

As a side note I’ve been doing this since 2011 and ive only ever been in 2 accidents. 1 door and the other was just me being drunk and running into a pothole. So no the traffic is not an issue.

As for locking I usually lock the wheel to the frame and I only leave the bike unattended for several minutes at a time when going into residential, restaurants.

In terms of the weight the road bike is close to 20 pounds the single speed is around 17. So the difference isn’t crazy they are both steel frames.

As for the shifting I do shift a lot. It’s NYC so hills, traffic, I have to change my cables about every 2-3 months, and get the shifters recalibrated.

I think the gear ratio is 46-20. I’ve lowered it over the years, but I can notice the hills especially if I haven’t been riding everyday. It’s a free wheel.

However I’m wondering if the stress to my knees is more of me not being in shape, and wondering if strengthening and stretching will allow me to use the single speed more often.

Also I’m in my early thirties, so it’s not like I’m at that age where I’m afraid for surgery or anything.