Yes and no. MBR boot can happen on GPT or DOS partitioned drives but you can only use EFI boot on a GPT partitioned drive. Note DOS here is the name of the old way disks where partitioned GPT is the new way. If you plan on EFI boot the partitioning method needs to change to GPT. If you can boot any Live Linux check fdisk -l it will tell the partitioning type
As it seems your system is running OK now, it is a bit superfluous, but nevertheless.
Deleting the partitions only results in an “empty” partition table. And that table will then be used when you create new partitions. As long as you do not create a new partition table, it will not change it’s type.
That may be an intelligent guess, but the partitioner shows the names, as tools like fdisk, gdisk, lsblk do. We rather like to see your copied /pasted computer text then your guess, because we trust computers more then people
And, btw, when ignoring the message gives you a functioning installation, a bug report may be something useful. The problem here is that it is only a warning (the installer seems not to be very sure about it and well, you can always warn to be on the save side). When you decide to file a report: https://bugzilla.opensuse.org/index.cgi
And please post the link to your report here so we can all trace what happnes with it.
No partitions does not change the type of partition table that determines the type of partitioning used. You would need to also wipe the first meg or so of the drive.
The expert partitioner should show you the order. The order is determined buy how they are plugged in and or the BIOS/UEFI
UEFI is the new BIOS. In the olden days (6 or 7 years ago) the default program that starts a computer was called the BIOS (Basic Input Output System) This is been replaced by a new standard way to start a computer UEFI (Unified Extensible Firmware Interface) UEFI can emulate BIS function ie Legacy booting