Hello Mark. Thanks for reaching out (all the way from the Philippines) and welcome to the community!
My church experienced difficulties on a consistent signal for our main displays as it fails considerably in the middle of our church services. Our current setup is HDMI-CAT6-HDMI which would be split into three displays - main projector, stage monitor and confidence monitor. This failing signal prompted us to consider upgrading our long runs into SDI cables.
Definitely a common issue with HDMI over Cat 6 extender systems. You've made a wise choice to switch to SDI.
I'm still studying the plans and review the technical information, but with this I also want to consult you, as an expert in this field, on what we could improve on our initial plans, what could we implement to make it better with a minimal increase in budget, and how we could future-proof our initial plan.
First of all, very nice wiring diagram! Well done.
The first thing I noticed was that you're running HDMI > SDI > HDMI and then splitting HDMI to go to the projector and stage monitor. That is not a method I recommend. One of the major issues with HDMI is that it does not distribute/split easily because it requires a constant 2-way communication between the source and display. When HDMI gets split it can weaken the signal substantially and/or confuse the 2-way signals causing the image to flicker or drop out.
Instead, I recommend going HDMI > SDI and then distribute/split the SDI signals before going back to HDMI at each destination. Below is an example of what I'm referring to:
In your situation I would probably recommend using HDMI to SDI converters with multiple outputs so you don't have to use a separate SDI distribution amplifier, though. For example, using the Decimator MD-HX at the computer and running 2 separate SDI lines from the computer to the projector and stage monitor. That will give you flexibility in the future as well, giving you an extra SDI line that can be used for different content or sending video back to the booth from the stage.
As far as converters, I would recommend using scaler converters at the computer. Those will help get the signal into a proper SDI standard in case the computer outputs a signal that isn't compatible with SDI. You can see my recommendations for those converters here: The Best HDMI to SDI Converters
In your diagram you have a single DVI cable going from the computer to an external box, then from that external box to HDMI and DisplayPort outputs. What specific box is that? How will the single DVI output from the computer be split to show different content on the confidence monitor than what is on the projector or stage monitor?