[LG V35 Questions & Answers] Android#:
First of all, these are LG V35 forums but that's fine I guess since the V35 and V40 are same exact when it comes to how to unbrick. You can't mix Android 9 and 10 like that, no wonder you got bricked. At this point, even if somehow you get into LG UP, make sure you don't use any options in LG UP or any other tool that you don't know about. Lot of people tend to use options like 'chiperase' thinking it will fix the phone but they make matters worse and permanently loose their IMEI and EFS partition info.

For LG UP you need download mode, not fastboot mode. Are you able to get to download mode by holding volume up button while the phone is turned off and by plugging in the USB cable that is connected to the computer? I think you should still be able to get into download mode even if phone can't boot but If you can't get into download mode this way and can't boot into android, the next option will be using QFIL/QPST by putting phone into EDL mode. Since the process of unbricking will be same for both V35 and V40 phones, I will briefly guide you here for others with V35 to see this info but if you need further help, it would be best to post in LG V40 forums. The QFIL/QPST folder and V35 engineering bootloader from link below work for both V35 and V40 but rest of files such as firehose and actual partition files are different between the two models.

First go to the LG V40 unlock guide post here: https://xdaforums.com/lg-v40/how-to/unlock-lg-v40-via-9008-root-t-mobile-t4042207

and download QPST, firehose and v35eng bootloader files attached to the above post and thank the OP. You can just follow that guide as if you want to unlock bootloader but basically you are going to put phone in EDL mode using that guide so you can run QFIL and access partition table using partition manager and flash the files necessary for download mode. All you need is bootloader and and laf partitions to get into download mode which are abl_a or abl_b (based on which slot you are booting from, slot a or b), and laf_a or laf_b. Important to note that laf_a and laf_b are same exact files actually, same is true for abl_a and abl_b. I am uploading the files for V40 on Mega as these were too large for XDA but if anyone needs them for V35 just post here and me or someone else can upload them:

https://mega.nz/folder/o2xEnCSb#W5icT64hASwKk67G9IngEQ

MD5s: abl_a: 3a9e506523e92b6a242af2e1cb5987fd, abl_b: 3a9e506523e92b6a242af2e1cb5987fd, laf_a: cf9edf4748f768cfa3c88ca362ff36b3, laf_b: cf9edf4748f768cfa3c88ca362ff36b3

After you download attached files, flash abl_a file to abl_a partition, then abl_b file to abl_b partition, flash laf_a file to laf_a partition and laf_b file to laf_b partition. You can use Xsavi's guide to put the phone in EDL mode, start partition manager, then use the option load image and flash the files to their respective partitions. I don't remember if these are from Oreo or Pie as I don't own V40 anymore but I think for unbricking purposes it shouldn't matter as you are going to flash a whole KDZ on top of these anyways. Once done, turn phone off, start LG UP on your PC, hold volume up button while you attach the USB cable to the phone and it should put the phone in download mode. Use partition DL and flash full KDZ either Oreo or Pie first before you try going to Android 10, best would be flashing Oreo first as some people have had problems going straight to Pie or Android 10.