@JJSEA , I'm afraid I'm not familiar with your motherboard or 5600x so can't lend any specific advice. I would say though, if you're not too savvy on clocking RAM, have a look through that reddit post I linked and do some background research so you can manually dial in your settings, it's the only way you'll achieve higher clocks and perhaps getting all four sticks working at said clocks. e-die can be a very good performer but it's not without quirks.
1.35V is totally safe, this is more in line with what commercial kits are set to. Server RAM is aimed at low power consumption over bleeding edge performance so bringing it back up to this level is no issue at all.
Some people report e-die to scale poorly with voltage above a certain (varies) limit and some say it scales all the way up to 1.5V (with adequate cooling). I think it must be rather system specific.
Yes, 1.35V will generate more heat but not enough to even warrant heatspreaders I would imagine. Longevity will take a small hit but unless you're planning on running them in your rig 24/7 for the next 20 years, I probably wouldn't worry about it.
There is a chance switching motherboard will help, but likely only on a superficial level in that the auto settings of the new board might be better suited to your particular set up. Wither way, you'll still need to get your hands dirty if you want to maximise.