Commutation is easy, just get 10 000 1kV SCR units, each with an optical triggering input, and stack them in series. Then use a really big semiconductor laser to provide the 100W or so of power required to drive those 10k fibre ends all at once, or simply split the load over a few smaller lasers and include some protection so a failed laser does not cause a cascade failure ( really bad at this power level, really not nice to be in the same building as bad), and duplicate 11 more times to get a three phase switch and commutation switch. 12 switches because you want to reduce the switching losses, and doubling the size of the transformer and the extra iron and copper is a very small part of the build cost to get a single pair of SCR and commutating SCR per transformer tap. Just need some small 10MV capacitors, some small resistors ( only a few meters long, so small) and a sodding big transformer, then some low voltage ( only 150kV rated) capacitors and inductors on the input/output side for filtering.
Incidentally most HVDC lines are a single bundle on the pylon, typically here a rope of 6 large aluminium sheathed cables that are held suspended by a very long glass insulator pair. They have regular spacers along the line so that there is little whipping in the wind, and a set of dampers each side of the suspension point ( those things that look like dumbbells sticking out of the line) so that the cable does not build up energy from resonance.
The return path is from a massive buried earthing mat at each station, so consider that as well in the line loss, it has to be massive so that you do not get a massive ground potential difference coming off the mat, and large enough that it couples to the local ground with low resistance. Requires regular watering in dry conditions or you start having large losses. Here there are 2 HVDC lines, one 200kV positive and the other 200kV negative, but they often run in reduced power mode ( failed line, blown up by war, no technicians to maintain the one end) so use the earth return a lot.
Also consider HVDC lines are inherently bi directional, they use the same stations at each end, as synchronous rectification is lower loss than a diode stack, and allows quicker turn off on high voltage trips, you can turn off before the end of the cycle by force commutating the active stacks much faster than the mechanical switches can start moving to do a physical breaking of the line, complete with the high voltage arcing that might take a second or two to break the current.