DIY Shower Reno Question - rough in valve - water mixing

[CJ Underwood]

I am renovating my small full bathroom (shower is only 3' x 3'), bathroom is like 5' x 5'.

I have it fully gutted and replaced subfloor where necessary. I have switched the plumbing from copper to pex and have the new rough in valve installed. When I turned the water back on the rough in valve I am assuming is mixing the hot and cold water so we dont have hot water anywhere. Is it OK to install the cartridge now before the tile is installed or how does one avoid this issue while doing the work DIY and not able to work on it 10 hours a day where its ok to not have hot water for 1/2 days.

I have shutoff valves where I made the copper to pex conversion but I need to turn the water back on... seal the wall up with cement board so I can continue and waterproof it, tile it, etc. Just not sure how fast I can get it done and dont want to not have hot water for weeks lol.

Probably an easy question but I cant really find any info about what people do to avoid this. The Rough in valve does NOT have shut off valves integrated on it.

Thanks for any input!