even if the thread is a bit behind now, here are my 2 cents on the topic...
after a few years of experience with the Manfrotto RC128, here is the summary in combination with the Kowa 883:
1. The quick-change system of the Manfrotto works and can be locked and released quickly and reliably, but unfortunately there is no way of balancing the center of gravity of a spotting scope, for example, by simply moving it back or forth.
Slidable quick-release plates of the "Arca-Swiss" type have a clear advantage here. They also snap into place quickly and - despite their ability to be moved - have integrated a protective mechanism that prevents them from accidentally falling out. In practice this means that you can quickly rebalance a balanced spotting scope as soon as you have attached a camera for digiscoping, for example.
2. The locking screw on the side of the RC128 does not allow an adjustment in which the spotting scope can on the one hand be tilted slightly, but on the other hand cannot accidentally tip forward if one is not careful.
It has happened to me and practically all other RC128 users several times that the spotting scope accidentally tilted forwards (or backwards). This can lead to the fact that the entire tripod falls over when the spotting scope is tilted (light tripods are particularly affected by this).
Because of these facts and that the RC128 is not exactly lightweight (728g without plate), I looked at numerous fluid heads.
My choice fell on the Sirui VA-5 among the fluid heads that use the Arca-Swiss system, have integrated a counterbalance and are also stable, high quality, light and affordable.
It weighs only 472g (without plate) and holds a Kowa 883 in position at any angle of inclination, even if the locking screw on the side is loosened so that the spotting scope can be tilted as smoothly as possible. Accidental tipping forwards / backwards can no longer happen due to the integrated counter balance.
the Sirui VA-5 is around 180, - EUR in Germany and is certainly not one of the cheapest heads, but currently one of the best lightweight fluid heads on the market IMHO
Cheers)