I have a Prusa MK3 with the MMU2, and I definitely plan on upgrading at some point to the MK4 and MMU3. Since I don't have any direct experience with any multi-material options other than the MK3 with MMU2, the only thing I will say is to ensure that whatever you buy is an open source printer. I cannot tell you how much of a difference that makes when there are so many incredibly smart makers in the community that can take full advantage of that to provide solutions for basically any problem. My first printer was closed source, and I had a document that was several pages long that broke down everything that you needed to do (and not do) to get that machine working well and upgrade it yourself within the confines of what was possible. Prusa support is top notch, as is their quality. I can't recommend the brand enough, even if I don't have experience with the latest multi-material options.

Note that I don't own a multi-filament setup so I can't speak from personal experience, but this is after I'd done extensive research into it and determined it wasn't worth it for my personal use case.

What kind of parts will you be printing in multi-material? The kind of prints you are doing will greatly determine the best approach. Some smaller prints with many filament changes will result in purge blocks with more mass than the part itself.. ie more than double normal filament usage.

IMO the tool changers are the way to go and I think we will be seeing more of them in higher end machines. I personally wouldn't use multiple materials for the sake of colors, but different tool heads offers the ability to print with different materials at different temperatures, as well as different nozzle sizes or different setups altogether. For example you can have a 0.6 volcano nozzle for the bulk of your print, a standard 0.2 nozzle with the same filament for fine details, and a 0.4 nozzle with a water soluble filament for supports.

This is just speculation, but I'm betting that we'll see tool changers with more than just extruders on deck - imagine if you could add a Dremel head for post-print sanding, or a drill bit to clear out holes.

My personal experience with a couple MMU2 on a mk3 at college is it's garbage. You have to sit there and babysit it as it will error frequently and the filament changer seems to need endless readjustment lest it jam with a filament sprig for the 55th time today. Maybe they are better if you customize them more and don't beat on them like college freshmen do, but... I like the idea but the ones i've used are not that fun.

Be careful of what other kind of filament changers you buy though. If you want to do multi material changes involving TPU, systems like the MMU where it's a single feed tube switcher seems to be OK, but integrated feeders like the Bambu Labs ams style will eat tpu for breakfast because it doesn't follow guided paths very well. I think? the ERCF can feed tpu fine?

I know this post is quite a few days old already, but I still wanted to add a bit to the discussion.

The printers you list vary wildly. Both in terms of design goals ("what is the printer meant to do well") and assembly requirement (from "ready to print in 10 minutes" to "you build this for like a week until something moves"). A Qidi is basically ready to go, a Prusa will take some time to put together (how much depends on if you got it as a kit or fully assembled). A Voron 2.4 takes about a week to build just for the printer, not including ERCF and/or tool changer, let alone tuning of said ERCF/tool changer.

Also there's the Troodon, which is a Formbot prebuilt that is closer to a real Voron 2.4 than a Sovol SV08, just to add to your list. It has a stock stealthburner tool head compared to the proprietary thing that Sovol uses, for example.

I've recently built a Voron 2.4r2 (Formbot kit) and loved it, but it was like my 4th printer (and a previous printer was a self-sourced scratch build). So do you have experience with 3d printers, and building them or tinkering with them? I would probably not recommend building one otherwise, but it's not impossible either, just expect a relatively steep learning curve if you have no prior experience.

Do you want to mostly just print in colors but same filament ype, or do you want to mainly have multi-material capabilities? So do you need 5+, or would 2 colors with the option to expand work for you?

If the Voron is a real option for you, I'd highly recommend it. Just make sure you're going with a can-bus based build/kit (like Formbot). These days I wouldn't go with an ERCF due to the complexity of building it and then setting it up, as tuning is supposedly a bit of a process. Also you mentioned that the amount of waste during multi-color prints is a real factor for you, and that puts single-nozzle systems inherently at a disadvantage as you just have to purge the hotend on every change. So I would suggest a tool changing system, and I would either start with that (but just 2 tool heads), or add it as the first project. Specifically, I would suggest using the Tapchanger as a modern system. Frankly adding a tool head like that is much less effort than building an ERCF, but also just adds 1 filament each and not like 9 at once.

I can't speak to multi colour perse, however the creality K2 went on sale today for certain countries and they have a 50% off sale for a limited number of units, they sold out today's stock but will have it again tomorrow then they will drop to 40% then 30%. Might be good if your willing to be a beta tester essentially.

The Prusa units seems to waste less filament.