I've run a .5 mV cart through my ZP3 and followed by the CSP3 with success.  (I was waiting for my SUT to arrive.)  The 2.5 mV will certainly work but ZP3 output will be lower accordingly, so input into your SE84UFO will be below optimum.  I think you'll be well below the 1.5 V input needed to fully drive your amp.  If you currently have plenty of headroom, you might still be okay.  However, a gain stage, post ZP3, may be needed.

Can you replace your stylus?  Or are you itching for a new cart?

See my profile below for which cartridge I use with my ZP3 and no SUT

So I ended up getting the SS zephyr mk iii es and it’s a fantastic cartridge... incredibly resolved, great dynamics, and pairs perfectly with the VPI arm. Plenty of output for the ZP3 and still plays very loud through the SE84UFO and 100db efficient speakers. Have not needed to go past 12 on the amp to get very loud output. This cartridge is only 2.4mv output, but almost seems louder than my previous 4mv output cart. This is probably because it is significantly more dynamic, but I usually find myself playing at 9:30 vs 9 previously on normal listening.

It does bring up another issue of gain vs volume. Like Archie said, I’m far below the 1.5V optimal input voltage for the SE84UFO going directly from the ZP3. There is definitely a sense of “heft” missing from the sound, but this was also missing from my old 4mv cartridge. Plenty of volume without hitting the ZP3 noise floor, just not enough body. My Schiit Modi DAC has an output of 2V and doesn’t sound as good as the vinyl rig, but does have a sense of scale to the sound that vinyl is missing, even at same volume levels. If you’ve ever recorded an instrument or vocals, boosting the gain will drastically change the character of the sound and make it “bigger” at same volume levels. Same situation here. Even if using a very HO MM cartridge like the Ortofon 2M series, im pretty sure you’re still below the 1.5V full output for the SE84UFO. I know this is discussed in other forums, but I think it’s worth mentioning this is more of an issue with the amp rather than the ZP3 as pretty much all MM phono stages have similar gain to the ZP3.

This has lead me to search for a Zstage or possibly a CSP3, however since the schiit is already above the optimal voltage, I really only need the single source voltage gain. If anyone has had a similar experience, I’m interested in what options there are to resolve this at a low cost

I’m sure the CSP3 would make both vinyl and the DAC sound fantastic and give a ton of body to the sound. I’ll probably own one of these eventually but the Zstage (buying used if possible) does seem like the best low(er) cost option. Also the single tube is nice for not getting into a rabbit hole with tube rolling.

Is it best to stick with Decware on something as simple as a single source gain stage or are there other companies/components out there that won’t jeopardize the great tube soundstage of the ZP3? I haven’t really seen much in my search that does something similar to the Zstage with the same simplicity. I want to avoid getting some crazy multiple input preamp because when I have a need for that, I most likely will purchase a CSP3.

The ZSTAGE with its variable gain might be good (seems like a CSP3 light?) although I have no experience with one.  The CSP3 is certainly not the low cost option but I'd keep mine over my ZROCK2, which I took out of the chain a year ago and have no plans on putting back.  However, I've never tried the ZROCK2 without the CSP3.  In my current set up the ZROCK2 doesn't offer anything I like.  Remember, it shifts tonal balance, which isn't always wanted.

The ZROCK2 seems to just add gain in an increment of 3dB, I think.  If you want to "ride the gain," you need a ZSTAGE or like.