Holy water can be mixed with non holy water and it fully becomes holy water. Simply having a holy water tank that refills whether it becomes 2/3rds full gives you an infinite source of holy water.
We jumped in with 5x of the ender 3 s1s. I can't recommend them enough. After very little calibration and upgrading to a pei bed, they ran flawlessly for ~18 hours a day for 2 months straight.
You would buy the foam and drywall separately. You can either glue the entire thing together or glue the foam and use tapcons (concrete screws) to attach the drywall to the wall (screws go through the foam) if you want to do something without giving up space... hang a tapestry? lol
Not in the eu, so translate the material names as needed. Foam board insulation with a layer of plasterboard directly over the top seems to be your only option. If you are worried about durability just use a thicker plasterboard. Keeping the assembly under 5cm might be difficult if you want a useful r-value.