Yes you can do that. I experimented with for some time and while the trainer did change the resistance mimicking the course profile, the experience was rather rough and the gradient changes felt sudden and unnatural. I suspect that it can be very dependent on the quality and smoothing of the elevation data of the activity that you load. Then this function was broken by garmin firmware updates for quite some time, and although I believe that it is again working I have not tried it again, so it could be worth to try see if they have improved it somehow (I doubt it and I would not recommend you to purchase an edge computer just for that). For the purpose of training for a particular event I found that I could do that much better with the Bkool software, as they converted your .gpx activity in a virtual route that did indeed feel pretty natural, especially if the source ride data was of good quality. My times in these virtual routes were reasonably close to the real thing at comparable effort levels. This was last season and things may have changed, you may want to give it a try. I think that they have some limited access for free and a 1 month free trial.

On a second thought, since you are a zwift user you could also try converting a route into a workout using their tool https://whatsonzwift.com/gpx-to-zwift-workout/ . Not the same since you are in ERG, but on the plus side you can input the intended intensity factor and then experiment how slammed you will be at the end of the session at different effort levels. In summary, stop looking for excuses and buy yourself the 520!