I carry my bikes up and down only 7 steps, so my case is quite different from yours. I never had much trouble carrying my gas engined bikes. When I went electric is when the problems started. My first e-bike is a heavy steel framed Schwinn and I built it STRONG with no concern for weight. Second e-bike I learned a little and made some attempt at lighter weight without sacrificing strength. It has an aluminum central frame, but steel suspension forks front and rear. I replaced the steel pedal cranks and steering head with aluminum. It is still a bit of a strain carrying it due to the 17 pound battery and 17 pound direct drive hub motor.
E-bike #3 is a work in progress and I am making every effort to go as light as possible, yet retain, even improve, strength. I am going with an aluminum framed Genesis Saber 2600 mountain bike for the strong but light alloy MBT frame. The quality of the bike is very high and is still only $200.00. While the rear suspension fork is all aluminum, the front suspension fork has steel lowers and the fork alone weighs 5 pounds. I am replacing it with a higher quality alloy fork unit weighing just under 3 pounds. I will be using a geared motor that weighs just 7 pounds instead of the 17 of the direct drive hub motor at a loss of some power and speed, though the battery will still be the Ping LiFePo4 battery weighing 17 pounds. I will not sacrifice range for lighter weight. I also replaced the mountain bike front gears with road bike gears so I can go a lot faster pedaling than the low geared MBT can go. Road tires are smoother riding and lighter than nobbies, so the tires got replaced. The wheels were also replaced with higher quality, stronger and lighter rims.
The new e-bike will actually weigh a tad less than my last gas engined bike, although just the base bike will cost about twice what the basic bike cost before I put the GEBE-SR EHO 35 engine on it did. Carrying it up and down stairs will not be a great strain. Light weight, strength, and quality can be built into a MB, but it does cost more.