In this game you want to promote as soon as possible. Engage keeps track of internal levels, but unlike past games does not automatically set your internal level to 21 upon promotion.
In say, Awakening, a character in their advanced class who was promoted before level 20 gains EXP at a slower rate than they would if they were of the equivalent level in their base class (1:11, 2:12, etc.), so you have to balance the benefits of promoting (immediate stat gains, new weapon proficiencies, etc.) with the downside of lower EXP gain.
In Engage, your internal level is whatever it was before promoting, so the rate of EXP gain remains the same. Furthermore, because the growth rates of advanced classes are almost always higher than their base class counterparts, you'll actually stunt your units' growth if you wait until level 20 to promote.
Both master seals and second seals are consumed upon use, so the decision making in this game isn't when to promote, but rather who to promote first, as both kinds of seals are limited at first and only become unlimited later on.