Me too, I'm both IT with a specialization in networking (and further specialization in wireless), but I'm also a qualified amateur radio operator (ham radio).

To say I know wireless bands and constraints with available frequencies, contention, interference, scattering, attenuation and free space path loss, is an understatement.

Zwave and ZigBee, at the time I was making the decision to go one way or another, about two-ish years ago (maybe a bit more), were fairly comparable, and the set of what was available was fairly equivalent. This was before matter/thread were barely a concept, and long before anything thread/matter compliant was on the market. So I weighed the options based on a few factors and one of the more important factors that went into the decision was the 900mhz band that's used. Zwave now has 2.4ghz, I don't remember seeing any 2.4ghz support on zwave at the time.... That was so important because of the interference that ZigBee would have created, and suffered from, with the WiFi in the house. We have 7 access points in the house and plans to add a couple more. Not all of them are broadcasting on 2.4ghz for the same reasons, but still, it's a lot of activity getting crammed into a fairly small band.

Bluetooth is already on 2.4ghz, so we're already going to hit some interference, plus all the problems we are likely going to experience from neighbors.

2.4 GHz is a really small band, around 72 mhz wide in total (from 2401 MHz to 2473 MHz). While 5ghz is more like 745 MHz (5150 - 5895 MHz), with some caveats due to regulations. It's still nearly, if not more than, 10x the channel width, depending on regulations.

I have band steering on, but we have some older IoT stuff, mostly smart speakers, which are 2.4ghz only, so we still need it.

To say our houses 2.4 GHz is occupied, is an understatement. We need to keep that band as free as possible, and zwave had the right specs to make it happen. Then the entire home automation community seemed to pivot almost entirely to ZigBee, thread, and matter, running on 2.4ghz. sigh.

Not to mention that microwaves run at 1000W of power or more, at 2.45mhz with only a poorly built Faraday cage to protect the airspace. I try to make sure that the microwaves in the house have good isolation, for safety and communication integrity, but still, that doesn't matter if the neighbor uses their microwave often and doesn't care if the signals are properly isolated. Even 1/100th of the power leaking out (around 100W) is 100x more powerful than most wifi access points (which usually sit around 100mW or 0.1W of transmit power)...

2.4ghz is a mess. I don't want to use it, but I can't avoid it.

And everyone seems to be dogpiling stuff onto the band for no discernable reason. 900mhz is pretty "slow" in terms of bandwidth, but how much bandwidth do you need to tell a lightbulb to turn on, or have a device report that a button was pressed (for a light switch for example).

It doesn't make sense. Everyone seems hellbent on making 2.4ghz their go-to, and not understanding why that's a terrible idea. 900mhz has better penetration power, and more than enough bandwidth for the task. Use it, FFS.