I am guessing this is being posted in relation to the virus. Is this likely to be at all helpful? Is having a strong diaphragm useful when your lungs are filled with fluid?
Maybe an MD will chime in, however in case not imo the diaphragmatic training is only beneficial for singers and yogis and others who need to do large diaphram style breaths. Most people will go back to their normal style of breathing even if they're trained and practice diaphragmatic breathing after they're done concentrating.
If I had to recommend one thing to buy for a regular person to use, it would probably be a really simple inspiratory/expiratory trainer. Basically you inhale and blow through a hole and it strengthens your lungs.
This one is low tech and bombproof, cheap, easy to disinfect and used a lot in the field: http://www.medi-careequipment.com/Catalog/Product/26832/Pfle...
https://www.respiratorytherapyzone.com/best-breathing-exerci...