I’m an older person, so my advice may not be current.
I would tend to phrase this differently.
“I thought it would have been more fun” 3rd conditional, after the event in the following, “...if I’d gone there alone.”
In other words, thinking about it after it had happened, you believed that “it would have been more fun if I’d gone alone.” The verb in the second phrase is not a conditional.
”I thought it would be more fun if I were to go alone.” This was an event that hadn’t happened yet, so you had been thinking (in the past) about something that may happen at a future point in time. These verbs are 3rd and 2nd conditionals.
Most people I know would say, “I thought it would be more fun to go alone.”