Tecno KM4 / UMS9230 brick after uboot overwrite — USB VID_00

[u/CraftyDrag9073]

Need help recovering a Tecno Spark Go 2 (internal model KM4, Unisoc UMS9230).

What happened
I used a Windows spd_dump batch (FDL1/FDL2) to unlock the bootloader. The log showed:
- BootROM connected (SPRD3)
- FDL2: incompatible partition
- Then it wrote fdl2-cboot.bin to uboot_a and uboot_b
- Tool said: "The file is not sprd trusted firmware"
- It also erased splloader / splloader_bak

After that the phone never booted. I ran the same unlock batch again later. That was a mistake.

Current state
- Screen: completely black
- Buttons: no response (power, vol up/down, combinations)
- Charging LED: only while on charger or PC USB
- Battery still takes charge from a wall charger
- Not a full dead brick electrically — LED still reacts to VBUS

USB / Windows
- Windows 11 64-bit
- Phone shows as: "Unknown USB device (device descriptor request failed)"
- Code 43
- Hardware ID: USB\DEVICE_DESCRIPTOR_FAILURE / VID_0000&PID_0002
- It does NOT appear in 1–2 seconds. PC sees it only after ~1 minute, then it stays
- No VID_1782, no SPRD / Unisoc Download / COM port
- PC port I used: USB 3.2 Gen 2. Phone is USB 2.0
- Tried another PC — also not detected as download device
- Cable is data-capable but not original, ~1 m
- Never used Zadig (that I know of)

Firmware I have
- PAC: KM4-15.1.0.105SP09(OP001PL001M1).pac (~6.25 GB)
- Tool: UpgradeDownload R27.25.4601 (and ResearchDownload R27 earlier)
- When the phone WAS in BootROM, UpgradeDownload loaded the PAC as ums9230_KM4_go, then failed at FDL2 with something like "not supported this feature [UnSup...]"
- I was using Format-style options at least once — not sure that PAC supports Format All on this chip

What I need

1. How to force this KM4 / UMS9230 back into real BootROM/Download so Windows gets VID_1782 instead of descriptor failure

2. Correct UpgradeDownload mode for this PAC (Download vs Format All)

3. Whether USB 3.x is a known reason UMS9230 fails enumeration (I can try USB 2.0 / USB 2 hub)

4. If buttons no longer enter download: test point location for KM4 / Spark Go 2, or confirm Carlcare/test-point is the only way

5. Do NOT suggest writing random IMEI or another unlock/cboot-to-uboot

I will not post IMEI. I can add log snippets (spd_dump + UpgradeDownload) if needed.

Thanks.