Hi. Is there any reason that you need to do the single supplement thing at all? Just because you are a single traveller doesn’t mean you need to be alone.
I have travelled in the past and never bothered. I have met some really great people sharing tents and in dormitories. I realise some people are very private and quiet and it could be tricky for them, but meeting people is a really massive component of the travelling experience.
Croatia is awesome, by the way. I managed a trekking holiday there between the mad lockdown nonsense and it is a beautiful, friendly country with loads of outdoorsy adventures to be had.