While the device appears in Device Manager under "GARMIN Devices" it is in preboot but probably only accessible by Updater.exe when 'System' is on the screen.

Let's get you fully cognisant on what Cure firmware does so you can nut out if it's Cure or original fw needed here before going further. A cure fw, whether made manually or by the Cure3 application, is used to re-enable Mass Storage Mode solely for the purpose of removing likely faulty files, or re-formatting the device if bad files are numerous or unknown. So, let's examine the circumstances of your device becoming unusable. It wasn't immediately after any updates via GarminExpress/WebUpdater/MapInstall/BaseCamp, or after manual transfer of a files like GPIs for instance, was it? It was after you left the device in your truck and its battery flattened, cutting power to the device which would have switched over to its internal Li-Pol and then also flattened that. When you reconnected it to power, it started immediately started to bootloop on first start attempt, correct?

So, on balance, although there could have been on-board file corruption caused by the power loss, i.e. stuff in the visible file system held in region 83. However, because that's a rare occurrence in a healthy device, by and large it seems more likely that the device is boot-looping because of a firmware failure (corruption of the main system software in region 14) due to an aborted boot attempt due to another power loss when it was first started (after all, the internal battery was probably dead flat and maybe there was an brief interruption of power or the vehicle voltage was low.

If you now understand the above you'll grasp that there is no point loading Cure fw if it's a software problem. The fix for that is just to load the original firmware by Updater. If that doesn't change anything after a successful flash then we can look further, use Cure to see if that re-enables MSM.

If loading original doesn't fix it, gathering some more info on what's happening during the boot cycle would be helpful, so watch the screen closely when you power it on. Note the message/s that do appear at the base of the screen before it boot-loops again (e.g. "Loading Maps"), or observe if it aborts quite fast without any message displayed.

As for flashing in preboot, it can sometimes be a problem when devices are bootlooping because there can be things going on the device in the background which interfere with it holding preboot. However, usually persistence pays off and if you can't get the flash to start, just power off completely and start again. Sometimes dozens of attempts are needed however once the flash is properly and fully initiated and data packets are exchanged the flash should finish provided the device doesn't have hardware damage. So some more info to absorb, i don't want to needlessly concern you but as you're used to doing related Android and Odin flashing you'll get this i'm sure. There is another more sinister reason for a device to bootloop and that's because its flash chip is failing. Unfortunately Garmin's recent devices seem to be more prone to physical flash failure than the older ones and maybe they're using cheaper/lower quality chips lately, but whatever the reason it's not a rare occurrence in recent units like before. If that's the case then it'll be impossible to flash it whether in prepoot with Updater or by any other method such as microSD. Just as with a failing media card, sometimes a failing flash chip in a Garmin device will partly operate but eventually it'll die completely. Often doing numerous flash attempts will hasten that it seems, maybe there's more stress on the chip when flashing is the reason. Regardless, you have nothing to lose by trying to fix it using software methods and until all such options are exhausted i'm happy to keep trying.

BTW, you might be able to force the device off without disconnecting the battery each time. Try holding the power button down for around 30 seconds until the screen goes completely black, some devices need less time though. Unfortunately some will just immediately power themselves back on to start bootlooping again and that can be sign of underlying device health problems too. The battery isn't usually an integral part of the power circuitry so if that's the case with your DS51 too you can leave the battery disconnected and use only external power.