You can get a managed 5 port gigabit switch for as cheap as 20$, even cheaper if it is unmanaged.
On normal operation the SD card is not written at all. All logs go to tmpfs. Also the SD card makes the RPi un-brickable. You can always take out the SD, flash a clean image, insert, restore backup, and you are good to go.
a) IMHO USB3 for LTE, while nice, is a bit unneccesary. The maximum speed of LTE is about the same as what a USB2 port can deliver, around 300 mbit. And that's the absolute best case scenario.This is somewhat not-completely-wrong-but-almost-certainly-not-completely-correct.
b) What do you expect from "load balancing"? Last time I checked, real "balancing" between two lines (as opposed to failover behaviour or policy driven switching) is still extremely hard to set up with OpenWrt This is also wrong. Yikes!
c) I personally did this last summer, out of necessity: I used an otherwise outdated, dual-sim Android phone tethered to my OpenWrt, and had it decide which LTE connection to use. It was dead easy to set up, allowed me to position the "phone" in a convenient place, while it simultaneously received its tiny amount of power through the USB connection as well. I can't personally compare it, but seems like such a solution would do just as well as an USB LTE modem -- and you'd only need one USB port for both LTE connections.
The USB 2.0 port on my GL-X750 is hardware limited to about 80Mbps and not sure if this is the case with other hardware.
For load balancing, I'd like to have both failover capabilities and just general load sharing so I don't use all of the data from one sim. I am ok if I have to do this manually and map certain devices to a specific wan.
I am aware, but I am under the impression that load balancing with mwan3 will either happen as failover or with policies, not in an "automatic" round-robin way. This is, again, wrong. I win no points today, but at least I'm getting used to the taste of crow.
What I liked about my setup (admittedly jury-rigged, it was a makeshift solution for a two-week network outage, and I was very happy about the 10 or 15 MB/s it could still pull) besides how easy and beautiful in its simplicity it was, is that the phone itself was set to cap the data per SIM card, and it also handled the failover by itself. But I really didn't want to claim that it is a "better solution" or derail your thread, please carry on.
Load balancing (without cooperation from the involved ISPs or using a well connected remote server to stitch back together the traffic) remains a problematic topic (even on highend proprietary business routers), it kind of works to semi-statically distribute multiple clients over the available lines, but does more harm than good trying to accelerate individual clients (http/ https sessions breaking, triggering attack defences of web servers and services).
You might have problems getting enough power out of any single device to power up two LTE modems. You may have to use USB M.2 adapters that have dual usb ports (data+power and power-only) and use external power supplies in the power-only USB port.
A USB3 port can supply a max of 900ma, which is 4.5W at 5V, and LTE modems draw almost that much max (check your modem's specs)
I think a RPi4 could it IF you can get max current from both USB3 ports at the same time, which I'm not sure you can.