As you've mentioned data volume is huge. Then its gonna be a little issue going forward.
First of all you have to size your dashboards according to your devices. Then I'm assuming you've created your dashboards using published data sources.
If so then first connected the published data source in a blank tableau desktop and make and extract. Save the .tde in a path. If its a huge data source then it'll take time to take the extract. Once done then close the tableau desktop. Now open your published dashboard in tableau desktop. Then connect data source (extracted .tde) and replace the data source. Once your replaced the data source. If you've used any color KPI on your measures then color will be changed that you need to fix manually. Once done the formatting now saved the .twbx file. This file you can share with your end users. This file can be opened in tableau reader.
Let me know If this help.
Is the only reason the workbook/dashboard is going offline is because it takes so long to load? If so, have you considered looking into what processes are taking so long and explore the options to find alternative ways to query the data?