Peter Frank said all the key points.
Lower volume boards are easiest imo. Have switched from a really low volume wood board to a foam core board and it's taken some time to get used to, but mostly its about learning how a foil moves through water and how to swim things into place using the foil to your advantage.
Helps to really memorize where you plant your feet on the board for perfect take off. Learn to ride the foil even when the board is underwater or on its side in the water using just a bit of kite assisted foot pressure. Your always looking to line your COG up directly over the mast and use your foot pressure to maintain the angle of the mast and drive the foil where you want it to go. Eventually the spot you grab the rail is consistent, and then the spot you put your back foot becomes consistent. You end up with a pretty good feel for where your foot is in relation to your grab and it all starts to happen the same way every time.
Things to think about in getting away from using a "handle" to position the board.
Your always "swimming" the board into position. In doing so, everything is fluid and progressing. you don't hold the board in one position any more while you get a kite into the right spot and waterstarting from a dead stop. Its a choreographed swimming of board and kite through the steps in progression with no static spot where your holding things. It's one big long move from approaching the board to riding away where you are already "riding" the foil while in the water as it moves into the waterstart.
To do this you have to get away from muscling your board onto its rail with a forearm or worse your knee. This is hard with high volume boards, but once you get any foil swimming forward you can angle it so the mast will "swim" the board onto its side. When I drag myself to the board, there is a plan upon approach to waterstart into the opposite direction. Try to use the pull from the kite to get the board and yourself moving slowing on one tack, then use the kite's pull and the foil itself to help swim your board onto its side with the foil coming up to the surface on the upwind side. Takes little effort with a grip on the rail somewhere aft of the mast near the upwind back corner of the board. A tiny combo of slightly sinking the tail and angling the nose of the board into the wind will angle the mast to swim the foil up. As the foil surfaces to the upwind side, begin to U turn the board and bring your back foot up to plant in that familiar spot in relation to your hand on the rail, still using the kites slow pull to pressure that back foot onto the board, Push the board through the U turn that will see you pressuring both feet into the board for the back half of the turn and into a nice controlled waterstart onto the opposite tack. If you drag up to the board on starboard tack, you get up and go on port and vice versa. Kite redirect is coordinated with foil positioning and body getting up over the foil as the board completes the turn and the kite is coming across/down through the power zone and onto the other tack. ie, sort of a transition from dragging up to the board on one tack and riding away on the other with the start in between during the transition. Did I complicate that enough?
Advanced technique: Get the above figured out and its crazy how many weird positions you can learn to watestart from. Eventually as mentioned using only your feet and not touch the board with hands at all. Still figuring it out with the new board, but its all doable. Getting the board into position takes a slightly different technique. Instead of grabbing the leeward rail at the back, the foot will sink the rail from the opposite side. The mechanics are all the same, but the spot I put my foot to angle and swim the foil into position is different as you have to push everything into place. As long as the kite has a little pull to work against, you can sink a rail and get that back foot into position to control the board and swim the foil where you want, get the front foot in place, get your COG in line and go. Straight downwind, toe side, with the board on almost any angle, as long as your lined up, you can pop up to foiling. The only time I absolutely have to use a hand on the rail to "hold" things in place while I get the kite in position is in the absolute lowest of wind where it hard to keep the kite in the air. There is no extra pull from the kite to work with, and you have to use absolutely every newton of force just to get up.
What has mostly helped me get consistent with this stuff, is insisting on always trying to ride out of every mess up no matter how bad. As long as there is a toe on the board, you can assert some control on where its going. Many many many failed or partially failed transitions, starts, whatever, where the board sinks, or you fall back into the water, but your feet don't entirely lose contact, are recoverable. No matter what, keep driving that foil back into position underneath you. Use kite pull to press your feet into the deck and swim that foil into a position you can get balanced on. From there power up and ride away. I have done this on surfboard for years and it has saved me hundreds of trips into the beach to collect a lost board. At worst you fall right off, but mostly you end up half falling and contorting your way through some form of recovery. Eventually this philosophy helps you recover from most random miss haps, sometimes before they even really happen.