(1) That's great that no calamity has occurred as yet. I'm overjoyed.
(2) I did mine a few years ago. I don't recall that error message, but my existing installation didn't have extra partitions. I had already deleted Dell's factory recovery partition & my own leftover Win 8.1 recovery environment partition. Seems like you
made the right choice to click through it.
(3) I don't recall version numbers on the boot choices. Both of mine said "Windows 10" until I renamed one. Boot to 1909, then...
(a) Let's see a BCDEdit from an Administrator Command Prompt...
Microsoft Windows [Version 10.0.18363.836]
(c) 2019 Microsoft Corporation. All rights reserved.
C:\WINDOWS\system32>BCDEdit
Windows Boot Manager
identifier {bootmgr}
device partition=\Device\HarddiskVolume1
path \EFI\Microsoft\Boot\bootmgfw.efi
description Windows Boot Manager
locale en-US
inherit {globalsettings}
badmemoryaccess Yes
default {current}
resumeobject {7f582e76-a2f7-11e9-86db-c6c7375e4034}
displayorder {current} <<< You'll have two items under displayorder
toolsdisplayorder {memdiag}
timeout 30 <<< The boot screen lasts this long till the default loads
Windows Boot Loader <<< You'll have two Boot Loaders
identifier {current}
device partition=C:
path \WINDOWS\system32\winload.efi
description Windows 10 <<< This shows up on the boot screen & in MSConfg
locale en-US
inherit {bootloadersettings}
recoverysequence {7f582e78-a2f7-11e9-86db-c6c7375e4034}
displaymessageoverride CommandPrompt
recoveryenabled Yes
badmemoryaccess Yes
isolatedcontext Yes
allowedinmemorysettings 0x15000075
osdevice partition=C:
systemroot \WINDOWS
resumeobject {7f582e76-a2f7-11e9-86db-c6c7375e4034}
nx OptIn
bootmenupolicy Standard
C:\WINDOWS\system32>
(b) Type MSConfg into Search, & show a picture of the Boot tab...

(4)
(a) How do you like 1909?
(b) Have all the drivers seem to have installed?
(c) Can you do an SFC?
(d) Can you make a Windows system image backup?
(e) Do your programs seem to install & run well in it?
If your answers are mostly yes -- especially to (d) -- we should redo the clean install, this time wiping all the partitions at the "choose where to install" screen. You'll select them one by one & click "delete". Then select the huge unallocated space (which
will be all that's left), & click next.