The smaller, the better, the HX Stomp format is fine. Budget is flexible.
I'm one of the few people using fractal products here (I have the axe FX 3, mainly for bass) and the fm3 may be worth considering for your use case. I also have owned a helix floor and stomp in the past. The main reason I went with the fractal is I think it has better amp sims, but that's probably not relevant for your case, so here's some other random thoughts.
The big advantage that fractal has for bassists IMO is the much more flexible routing grid. It's very easy to setup whatever path you may need, where specifically with the stomp, having only two signal paths means you're very limited on options. As an example, if you wanted to use a guitar OD on the helix stomp but have a clean signal blend, you'd have to run the guitar drive on the second path since it doesn't have a blend. But now that you've done that, you've locked the second path into being your drive channel and you wouldn't be able to run anything else in parallel separately from the drive loop. I don't think this is a contrived example, as I currently use the horizon precision drive with a clean blend into an SVT as my main gigging preset for my Metallica cover band on my fractal. For my main gig preset on the 3, I have out 1 as my SVT through a cab sim for FOH, out 2 as a compressor only DI signal for FOH,out 3 as a post SVT pre-cab sim signal to a stage cab and out 4 as the SVT sent to a second cab block so that I can tune the sound differently for IEMs (we're still figuring out our live setup so having the option to do IEM, stage cab or both is great)
I also think that fractal has better compressors. For envelop filters, helix will have a much more friendly experience out of the box, as I believe it takes a bit of effort to setup an envelope filter on a fractal unit. I've never done it myself though.
If you have the budget and are willing to learn, I think the fm3 is an extremely solid choice. It definitely has more of a learning curve though. I've seen them going for as low as $700-800 used, so they definitely can be had for a good price.
If you're looking for more of a plug and play experience, the hx stomp is probably the way to go. Also, if you prefer to edit on unit, the hx stomp is the better choice. Fractals PC editor is amazing, but on unit editing can be a bit contrived sometimes.