You could go with a single standard monster of the xp of the solo and it would be much higher level and throw it at the party for the same desired CR threat level of a single monster. But the action economy and math would be wonky making for a different flow of combat rhythm, the defenses and attack rolls would be high, the attacks will be more concentrated on single targets, and the system is not set up for that situation as a design goal.
A solo does not need to be the BBEG narratively.
A lower level solo works well as an interesting singular fight.
To maintain some of the flow and form of the standard design across fights you can also just downgrade across the board, use elite instead of solo, standard instead of elite, minions instead of standards, single minions instead of multiples.