The brute force method is to copy the work onto a second track under the first one. Use the SOLO and MUTE controls on the left plus track selection and apply full reverb only to the bottom track.
Use any the volume control tools such as Fade-In and Fade-Out or Envelope Tool and fade from one track to the other. The fade can be as long and complicated as you want. Pay attention to the bouncing sound meter while you’re working. If you’re close to overload with a normal track, adding reverb could push you over into distortion. Audacity doesn’t overload internally, but you can’t export the show like that. Correct the volume and then Export.
When you export the work, Audacity will push all the separate tracks into one show, unless you stop it.
There is a warning to this. If you have a noisy or damaged track, the damage will reverb, too. Home recording people run into this.
These two tracks are backwards, but you get the idea. In this case, the bottom track is the clean one. Makes no difference to the show.
Koz