Vertical vs. Horizontal scaling (move up or widen out?)

[ZenMusician]

wait until you are regularly crushing the tables at a particular limit until you start multi-tabling.

A+ answer. The crux is decision making; more experience gives
more exposure to situational play. Multi-tabling cuts down on
manual reads and can lead to betting action mistakes, etc.

Don't worry about being a "serious online player". Play the level
that feels right to you and take shots at higher limits to test
yourself and get a feel for different styles of play before committing
your roll. Once it becomes almost "boring"...this is the time to add
a table (1 at a time seems right).

Good Luck and post hands at your new level.

-ZEN