It's my third attempt to install Solus dualbooting with Windows 8.1, as always, unsuccessful.

First, I tried advanced partitioning. There is no way to do unless you use a third-party tool to create the partitions and then select them in the installer (need to reopen because it doesn't detect the partitions if it's opened before you create them). So, I did it. I've created 3 partitions:
1) / ~ 50 GB ext4
2) swap partition ~ 2 GB only
3) /boot/efi ~ 512 MB ~ fat32

Noticed it doesn't accept my /boot/efi partition, it tries to install inside the small Windows EFI partition which is around 100 MB and can't be extended.

All other distros let me create a secondary /boot/efi only specifically for Linux, so it doesn't mess up Windows or bloat the partition...
So, since it hasn't accepted /boot/efi, I deleted the partition and installed on sda2 (actual Windows EFI partition). It installed normally, except that when I rebooted... It boots directly on Windows. No GRUB detected to select Solus, tried to use ESC, it didn't work either... It was like it haven't been installed even telling it were.

Haven't noticed but the issues were just starting...
Then I tried again, but this time I used the first option, that detect Windows, but I was unable to dualboot because Windows EFI partition was bloated, full of stuff from the other installation...
Tried a lot of ways to remove the "trash" inside this partition, but it comes back after being deleted... Screwed up the entire partition... 100 MB is just too small, most of the distros recommend minimum 512 mb and remember Windows takes a good part of it.
I have tried the first option months ago but it failed that's why I've tried directly the other option that looks like 'advanced', but in reality its not, the installer can't manage advanced partitioning like ubiquity and calamares installer.

I tried different ways, followed the documentation and alternative tutorials on YouTube. None of them have worked, so I'll need to use only through virtualbox or move to another. It's my third attempting installing Solus and failing. You guys made it more complex and hard than Pop!_OS that doesn't even use GRUB.

I've tried to extend the partition from Windows but it doesn't allow me. I remember doing it long time ago and it borked Windows entirely, so I won't mess with this again. ☹️

I've waited years for you guys to make a proper installer, but it seems you can't or just don't want.

You create the ESP fat32 partition via gparted and assign it the boot and esp flags there. You only have to do that if you're dual-booting, because the Windows ESP is far too small to reasonable support both. You don't set the ESP during the advance partitioning section, that is for /, /home, and swap. It is done later in the "Configure the bootloader & hostname" stage.

I've waited years for you guys to make a proper installer, but it seems you can't or just don't want.

No real reason to be rude about it. We have other priorities.