I have to register and give some advice:
1. When buy camera and lenses, please think about buy-in price vs sell-out price. You may use Leica or Hasselblad with "profit" if you can "time the market." (ie: buy low / sell high).
2. Kiev88 + digital back is for fun, when you already have the Hasselblad V system, not the only option. Because the biggest problem is not the synch, but the reliability of the shutter. In digital era, it is easy to press shutter 10,000 times. The shutter WILL have problem, especially at 1/30 etc due to second curtain escapement. Here is the video for Kiev 88 shutter CLA:
3. I have the Hartblei + Phase P25+. The Phase back can't be directly mounted - you would have to file the film gate etc on the body. Even then, the connectivity may not be secured - please use tape as insurance.
4. Sinar back may not work due to its wake up method. I bought a Sinar Emotion 75 and tried, it didn't work. I didn't do much further test / workaround and just sold it.
5. The Hartblei can synch under 1/30 with CFV back; For Phase, you can use a shutter button - after all, it is a switch button. So don't need to think too much about synch method. Think about evenness of shutter travel and reliability - this is its Achilles heel.
6. For the "button" switch, you can buy Canon rs60-e3, or use the Phase synch cable that have a small wake-up button built in the wake up cable. Of course, there are 1 shot vs 2 shots (latency) settings on Phase. But once we know it is all about switch "contact" for 1 time or 2 times, we will figure something out according to the situation. Leaf shutter or X synch method is just convenience, not necessity.
7. I contacted Arax about the "digital back modification" many years ago. They could not convert X to FP synch, but the technician was able to add a contact point on the mirror - so when you do the mirror lockup, the back will be waked up, then you can press the shutter to capture the image. As long as the mirror is up (and touching the mirror contact point), the back is always active. It will go back to sleep only after you advance the film knob and lower the mirror (the total back's active time may be 3-5 seconds). So you can ask Arax to cheaply add this contact point on the mirror - tell him that Yongfei's Hartblei got this modification about 10 years ago, maybe he will remember
Here are some photos. The Hartblei is mine (you can see the tiny button on the synch cable). I use this Hasselblad 2000 found online to show how the Canon switch works - it will wake up the Phase back upon pressing the button, no need for shutter or X synch. I took the Hartblei and Schneider 60mm F3.5 on a trip, and it can take beautiful photos. But sometimes the shutter travel was uneven and unreliable.