This very old game wouldn't be such a pain if I could reload without having to quit to desktop and restart the game every time. Was a fast reload not implemented back then?

You can load if your whole party dies.
The first Gold Box game with in-game loading was the Amiga version of Curse of the Azure Bonds IIRC.
But it doesn't take long time to fire up DOSBox on a modern PC.
If you are really impatient (or one of these tedious save scummers) you can use a DOSBox version with Save States function.

First of all, I dislike the term "save scum", as that implies that saving is somehow bad, a notion that I highly disagree with.

Second, if you do use save states, keep in mind that they tend to save the state of the RNG as well. In many games, this means that if you perform the exact same actions after a reload, you will get the exact same dice rolls; if you lost because you rolled a natural 1 when a 2 would have hit, after reloading the save state you will get the same natural 1 if you perform the same actions (and the game doesn't burn random numbers in the background). This sort of thing isn't normally obvious (unless you have a game with a horrible RNG; the original SaGa (Final Fantasy Legend) is one example), but it becomes apparent when you start using save states (and is something that makers of tool assisted speedruns have to take into account).

(Also, what one player finds tedious another player may find quite enjoyable.)

I have set up Gold Box Companion for Pool of Radiance gog download. But I don't get any of the GBC perks when I load my savegame. If you know how, please help me to set up properly. I have Windows 10. I am able to open GBC window interface and it recognizes my save games, but when I click Search I get the following message: - ERROR: Could not solve base address. Could not match 2nd ID although 1st ID was found. Incompatible DOSBox / game version might cause this.

Mail me at the email address found in GBC's Help-file (you can open it by clicking the Help button in the GBC main window).

you can make the game run in a loop by doing the following

go into the game directory from galaxy
enter the POOLRAD directory
create a new text file and the rename it loop.bat
edit the file and include the following

:loop
start
goto loop

then go up one directory
edit the file dosbox_por_single.conf
change the line that says
start
to
loop

now run the game. If you try to quit the game by exiting to DOS it will just reload the game back in. In order to quit just press Alt-Enter and then close the DOSBOX window.

You can also do CTRL-F9 to close DOSBox while fullscreen.