What are the key design principles when building a space plane?

There are a few.

Wings provide lift and control, and having not enough makes you more vulnerable to shifts in fuel weight.

Thrust to weight is misleading, what is important is that you have enough force to move your mass not only against gravity, but against your aerodynamic drag. Most efficient, vertically launched rockets don't have this much trouble, but SSTO ascents try to get as much lateral velocity as possible while still in atmosphere.

Air Intake. A major issue with most designs, more air intakes will let you go higher and faster before beginning to burn precious jet fuel for the final ascent. 3 RAM intakes per Turbojet is generally acceptable, but very lightweight designs may be able to get away with 2.

The deathknell of every SSTO design ultimately comes down to finding a magic balance between your final altitude (Air Intake), rocket fuel weight to breach into orbit, and airspeed. Too much mass, your airspeed will suffer, but you may get higher with more intakes. Small SSTO's trade off mass for more speed (which slightly increases final altitude)

The last challenge is highly technical, but unique to every craft: piloting. The timing, ascent angles, and thrust controls make or break any design fit to see orbit. An avionics control module and good structural rigidity (plenty of struts, nothing should wobble, ever) will help immensely.

Use a single jet engine or place them as close to the center and as far back as you can.

As you climb out of the atmosphere, the jet engines will eventually flame out. The problem is that they do so unevenly and send your plane spinning. So place them center and back to minimize the torque.

As intake air runs low, gradually throttle back. The engines will require less air and will be able to give you some more speed and altitude.

Set up action groups to turn off unneeded air intakes at low altitudes to reduce drag and save fuel.

Place cockpit and SAS as close to the center of mass as you can to increase leverage, improve stability, and save electricity.

Scott Manley uploaded a video about aerodynamics in KSP. I would argue that stability is not a high priority for a space plane as long as you have SAS and don't run out of electricity. Keep your wings horizontal to maximize lift.

The general ascent trajectory is something like: as steep as you can until 13 km altitude, then level out and slowly climb till you run out of intake air (for stock Albatross 4A it's is about 25 km altitude and 1400 m/s speed), then switch to rockets and climb at 45 degrees to space.