Generally the most economical and easiest way is to cut in some quality dry cat food...
If you take your existing 19% feed and cut 50% with a good 30% dry cat food you get in around that 25% protein and you still carry over a fairly decent balanced nutrition profile... You can probably find even slightly higher protein content cat foods to cut in that would require less to bring you up to the same mark...
There are of course other options like meal worms, or high protein veggies and the sort, but if they start holding out for those 'treats' it could disturb their balanced diet overall, and some of those treats like meal worms come with high fat content, so you get the bad with the good...