While that Neosplitter says it's for MoCA, I'm a bit dubious of 2500 MHz splitters. MoCA only goes up to 1675 MHz and more bandwidth isn't always better. There are other factors besides bandwidth, such as port-to-port isolation. For MoCA, you want lower port-to-port isolation. Those high bandwidth splitters are often designed for satellite TV, which doesn't need low port-to-port isolation.

If you are only going to use 3 MoCA adapters, consider getting a 3-way splitter instead of a 6-way. Like this Antronix 3-way splitter. If you are determined to use a 6-way, then there's this Antronix 6-way splitter.

The primary PoE filter should go on the input of the main splitter. You can put a secondary PoE filter in front of the modem, but it may not be necessary. It may be needed if the MoCA signal interferes with the modem's ability to receive the DOCSIS signal.

Finally, you may not need the tester. You can use two MoCA adapters as a tester. ;-)

potentially using 2 2-way splitters (so that the room with the modem only has -3.5db instead of -~5.x if I’d use a single 3-way splitter)

FYI… 3-way splitters are typically offered in 2 varieties, balanced and unbalanced. The unbalanced approach is effectively the same as cascading 2 2-way splitters as you’re suggesting, with the low-loss output port having around 3.5 dB loss; and 7 dB loss for each of the other two ports.

Example: https://www.amazon.com/Antronix-MMC1003H-B-Splitter-Frontier-Formerly/dp/B07PMLCVHQ

(Note that the page refers to its as “balanced,” but that version’s model # is MMC1003BH-B.)