Please follow the advice from /e/ support first!
However …
When you get stuck with “Fastboot mode only” for the first time it can be alarming. However Fastboot is like the communication channel for flashing the device safely with latest platform tools. So that is the mode you need most.
Boot does not succeed, so /system is not bootable (for some reason) this is the space which we want to get populated by /e/.
The usual fallback when /system boot fails is Recovery mode, so we deduce here that Recovery is also compromised.
Another complication for this device is mentioned in the install instructions https://doc.e.foundation/devices/sake/install:
Tip: Outdated fastboot releases dropped legacy A/B support, so it might attempt to flash to boot__a / boot__b rather than boot_a / boot_b if you try to flash boot. In this case, you must update fastboot to a release newer than or equal to 31.0.2. Alternatively, you can manually specify which slot to flash to based on what slot fastboot failed to flash to. For example, if fastboot fails to flash to boot__a, you must flash to boot_a.
I would prefer to see this worded as 
or Alert.
Meanwhile please continue reading. Including the links in this thread from the start! This, I think, is the issue filed by @fraksken
The following thread reads as somewhat complex but may contain some insights for you.
That would work, I do not know the device, but you may find a hardware key combination to Force stop, usually involving a 10 second press of Power key. If this results in reboot, then back to Fastboot, you will still need to try to find the combination that is Force stop rather than Force reboot.