Can I use an absolutely flat tip design for the electrode?

Absolute flat tip of electrode does not exist in reality.

Even if they made the electrode with absolute flat tip by design, after just one weld, the tip face is deformed to have a natural curvature with a radius of R=50mm to R=100mm approximately.

The ISO Standard electrode forms are mostly with tip face curvature R=50mm.

If you design it in the software with an absolute flat tip, it is not possible to build up heat due to the large contact area in the beginning of the welding process.