Should splines be good enough or is there something better I should consider or maybe something to do in conjunction with that.
Just a bit on the terminology, a true spline runs lengthways in a slot milled into the face of both mitres, through all or most of the joint (a through spline or stopped spline respectively). A different joint reinforcement, where a piece of wood is inserted into one or more slots milled across the corner is now commonly also described as a spline but this is more accurately described as a mitre key. In the video you link to Matthias is using mitre keys, not splines.
Mitre keys are generally regarded as not adding greatly to the strength of the joint, while splines do provide a significant improvement. However, this is with keys and splines of conventional size.
The strength of each reinforcement is due to three factors: structural advantage, the thickness of the wood slivers used and the glue surface area. The splined joint being superior in all regards. However, mitre keys were originally quite small and done using veneer thicknesses, resulting in only modest added strength while the keys in the video on the other hand are far larger than conventional ones and also thicker. So they probably add as much, if not more, strength than a typical spline would, being of relatively modest width (25mm / 1" being a typical maximum).
If you use either sizeable mitre key or fairly wide spline, using wood of some thickness (perhaps above 3mm / 1/8" material) I think it likely that you will end up with joints that are strong enough.
Even saying that, if you would prefer to reinforce the mitres using a method that is faster there are a few I can recommend. And the first two are guaranteed to provide greater stability and strength.
These are L-shaped steel mending plates or plywood corner braces. They are simply screwed or nailed into the frame at the back and essentially make the joint completely rigid.
Last but not least, the simplest reinforcements of all: large staples or corrugated fasteners. If you don't have any philosophical objection to using mechanical fasteners both of these require no effort to install yet add hugely to the strength of the joint, as the metal would have to shear through or be torn free from the wood for them to fail. Some people consider them crude and ugly, but they work and I would argue that their ugliness is irrelevant as they are on the back of the joint where nobody will ever see them.