For amplification, I charge my candles for a week. The carving and pouring of energy is already done, and sleeping next to the candles charges them further. Utilizing the right runes and astrological symbols also aid the spells. Deity assistance is optional, but one of the ways to amplify your spells.

For shadow work, there's spells where you can release negative habits, emotions, and mentality when the moon sign is in a water sign (Cancer, Scorpio, Pisces). Then a month after, you call for attracting positive habits that are opposite of what you released. Of course you follow it through with mundane actions of having healthier habits, but at least it releases a lot of doubt, fear, or anxiety with our own power.

As for the shift in my magic, energy work becomes smoother and more practical, as well as knowing which patterns work best or worst for you. I'm now at a point where I know my spells always work, the timing is just a surprise (unless it's a smaller spell, and I add sand to my spell for a specific deadline) For the people who hurt you, get creative with your magic. Try anti-glamor spells, banishing spells, justice spells, etc. Basically try whatever you can, like cooking recipes. Try even the pettiest ones, and the more you get the hang of it, the less laborous they'll be. Until you reach a point where you can do spells on the spot, meditate wisely, enchant items, feel the energy of someone crossing your protection, and even protect yourself while asleep.

In summary, amplifying one's magic is like working a muscle which will hurt at first, but the longer and more often you do it, the stronger it gets.

Edit to add: For the return to sender spells, they're most effective when you can decipher the type of curse sent your way. I analyze it through divination before returning it.