There is a hidden value in Dorico’s preferences XML file that you can use to alter this, if you really want to. (It’s not currently exposed because in the fullness of time we expect to provide more per-export options for audio, this among them). First, quit Dorico. Next, find the preferences.xml file in %APPDATA%\Steinberg\Dorico 3.5 (Windows) or /Users/your-username/Library/Application Support/Steinberg/Dorico 3.5 (macOS) and open it in a good text editor, then look for the following lines:
<audioExportReverbTailSeconds>
<type>int</type>
<value>3</value>
</audioExportReverbTailSeconds>
Change the number in the line to the number of seconds you would like the tail to be. Save the file and restart Dorico, and future audio exports will add that amount of time as a reverb tail.