Countries have been "temporarily suspending shengen agreement" for tons of reason without putting the EU at risk. I remember being stuck in trafic over 1h when crossing the French border in the summer 2016, where between a football cup, and the terror attacks, the police was "checking the border" ( to be understood as, police kept only one lane open on the highway, and even as setup a tent and a parking so they can "randomly check" some cars.
I also remember covid time when travelling within EU was involving a lot of paperwork.
Not sure what's happening right now in Germany, but it's not the first nor the last time, that the open-borders are suspended.
I feel like if there is a border problem with illegal entries to the EU, we don't solve it with tightening border security of individual countries.
This is something the EU members have to solve together and maybe countries with less or no outside EU borders have to support the border security of those who have.
Send contingents of border guards to the outside borders to support those countries to strengthen their borders to non EU countries. An international EU wide collaboration on border security is needed in my opinion.