I agree with niflheimr in everything.
Add at least a second engine. Ion ships, IMO, should at least move at 3k km/s.
Beam warships should be even faster, so they can close with the enemy
fast!
Imagine an enemy mounting a 60.000 km range laser (which is pretty low-tech) and being faster than your DD. He can stay out of
your range but inside
his range forever, picking you appart with impunity.
Personally, I would scrap the res-50 sensor and put the mass into the res-1 sensor. This would increase the range you can detect incoming missiles
and serve as a short-ranged anti-ship sensor.
If you increase the speed of your ship, you will also need to re-design the firecontrol for your railguns, because the tracking speed of the guns will be either your researched tracking speed (2.000 km/s from the looks of it) or the speed of the ship, the guns are mounted on, whichever is HIGHER.
To do all this, you will probably have to reduce the number of railguns (don´t forget to reduce the number of powerplants accordingly.
I would _not_ reduce the armor. Beam warships are designed to slug it out at close range. They need all the passive protection they can get.
I´d also suggest to perhaps half the amount of maintenance supplies. 6 years of maint life for a ship with fuel for 22 days seems a wee bit excessive
As this is your first warship, it is ok. Just be aware that a max. weapons range of 20.000 km is not even knife fight range, it´s
inside knife fight range, basicly belly-to-belly!