I am in the same situation like you used to be. But for tournaments it is important the emblemed heroes to be balanced. Especially 3* ones. The most important reason being your defense, which will be constantly attacked by stronger opponents than it should be. Because every tournament, the defenses are automatically chosen and based on that TP, they are attacked. So, it represents a mistake to emblem only 1-3 heroes so much and not the others. Especially if they are slow and rarely used.
So, after almost a year of experiments, I’ve come to the conclusion the rare heroes should be stopped at +9 for the most efficient roster. Not more than this, to have also emblems for epic and/or legendary heroes. But still emblemed, for a better score in tournaments and easier attacks. With emblems or not, comes a day when the player faces 2900+ teams in attack. Would you prefer to face them with 2400+ or 2600+ like me?
For one, I kept till last month my 3* at +6 but I decided +9 for all is the most effective rare roster. And yes, I have emblems for epics too. And for legendaries, since all important ones are +10 - +12.
As for sorcerer, like I said, no 5* option. I emblemed a Sabina +20 and will remain so, no matter what. She is a great healer and dispeller. And I keep the rest of the emblems for the day I’ll have a 5* sorcerer. I do not put them more than +9 on my rare heroes, because would be a mistake, as I explained above. This is what I’ve done and I always seek to bring the best efficiency in my roster. And if I were you, I would let Sabina +20, balance the rare heroes (for easier tournaments and event challenges) and then I would emblem Nat.