I currently use a SJ modified for single dose with SSP burrs. I had fun turning it into a singled dose grinder and it has worked out well. There is nothing really wrong with it but I have the itch to try something else.

What would be considered an upgrade from that set up? Would something like a Niche be enough of a bump in grind quality/usability to be considered an upgrade? I mostly prepare medium espresso with some light sprinkled in. I also have a E37SD that I use for bulk espresso but find myself using a single dose grinder a lot for smaller samples of espresso. I'm not looking for the "end game" grinder but something to have fun with for a bit.

A few that I'm considering:

Modest price range:
-Niche
-Eureka Oro Mignon

Niche seems to be the standard here. I've heard the Eureka got off to a rocky start but perhaps some of that has been addressed? I don't have an aesthetic preference between them. Eureka seems to be a bit easier to get a hold of. It really comes down to conical vs flat burr. Since my Caedo is flat burr perhaps it would be more interesting to get the conical to experience the difference between the two types of burrs.

Higher range:
-P64
-Weber HG-2

Both of these can not be purchased right now, must be preordered. It seems the P64 is the "end game" grinder without a serious investment. I like the idea of the Weber's huge burr set but am concerned that I may tire of hand grinding. I watched JH's video and it seems to take awhile to crank through 18gs. Is the quality difference that much better?

I'm afraid the Niche or Oro would be more of a side upgrade compared to the SJ with SSP burrs. What would you guys recommend? Pros and cons of each comparatively? I'm also not against recommendations outside of the above grinders.

I moved from an SJE (slightly modified to single-dose better) with SSP red speed burrs to a Niche, briefly.
There was an instant improvement in workflow. The Niche is a pleasure to use, and retains only a fraction of what I'd typically see using the SJ - though I do still use a silicone icing bellows to get everything. With med roasts (when I feel like a comfort blend, my go to is Hatch Supernova), it produces nice full bodied shots at 1:2+ ratios. Taste-wise though, I'd call it a lateral move, as the clarity I got from the SSP burrs was significantly better.
I have limited counter space, so with the arrival of my Max last fall the Niche has been relegated to backup/cottage grinder, though I still enjoy using it when I can. Interestingly it did have an influence on my shot style-preference. In order to match the body I'd gotten used to with the Niche, I found myself pulling tighter ratios with the Max and generally extract at 1:1.5 or less.

I have no experience with the other grinders you mentioned.

Thank you for the response. It seems something like the Niche would be a decent move. Perhaps not an "upgrade" but a move to something with better workflow and with shot characteristics more in line with my daily drink. I'm pulling mostly medium to medium/dark coffees with notes of chocolate/caramel/toffee/comfort. So there aren't a lot of sharp/distinct notes to miss out on like there are in lighter roasts.

I'm really liking the workflow improvements you discuss. Are you able to go without RDT for the Niche? That would clean up my workflow quite a bit. Right now I dose into the cup on the scale, transfer to tray, RDT, transfer back to cup, dose into grinder, grind, brush the burrs, grind, and blow the grinder out. If the Niche is dose on scale, dose into grinder, grind, and then blow out then that would cut a significant amount of time savings.

The workflow on the Niche Zero is better than any other grinder I've worked with.

My kitchen typically runs in the upper 60s in the morning, with RH around 30-50%. With medium-light and light coffees, I did not use RDT. Weight out was typically 0.2-0.3 g short of weight in prior to using a puffer. With the puffer I was typically within 0.1 g. For classic espresso, either would likely be sufficient. You might want to use a puffer at least first thing in the morning or last thing at the end of the day, just to get that pinch of stale grinds out.

No RDT would help a lot in my case. Since I already have the E37, then it may be rather pointless to go with the P64. The only benefit I see with the P64 is workflow but I don't mind using the, perhaps superior, E37 for the occasional light roast that I come across even if I have to dance around the hopper. The Niche would be for the majority of comfort roasts that I go through.

I don't RDT at all, I just put up with a little more mess than I'd have otherwise. I have a Pallo (I think) grinder brush that I really like and a small Makita vac that's always somewhere nearby in the kitchen.
I have a nice little spray bottle that I used sparingly for a month or so till I started to get the odd bean sticking in the grinder funnel. When I wiped the funnel with a clean paper towel and got visible residue, I decided to stop pre-applying the solvent to my beans.

Milligan wrote: The Niche would be for the majority of comfort roasts that I go through.

Yes, it's a great choice for med and darker roasts. I wouldn't say it was particularly bad with light roasts, just more difficult for me to get dialed in - but that's true of light roasts with any grinder.
There are people here whose opinions I respect who happily use the niche for all roast levels.

I second the Eureka sentiment.