5-spice can be a bit much, depending on the blend and the ratios. The one I like the best contains cinnamon, star anise, clove, Szechuan pepper and fennel. Some contain ginger instead, as Edmund's does, and use black pepper instead of Szechuan peppercorn since they can be hard to find.
Because of the long cook time, the rub will mellow. A sugar addition (I like no more than 15% sugar by volume (not including the salt) for butt) will also soften flavors as the sugar cooks. An addition of granulated onion and garlic helps too as their flavor profiles will dilute the more pungent elements of the 5-spice. Also, the teriyaki injection will mellow things dramatically.
If you use a 5-spice that does not contain ginger, then I'd do something like:
1/2 part ginger to 1 part garlic to 2 parts onion to 3 parts 5-spice, mix, then add 1 part sugar. In practical terms this would be
1/2 tsp ginger
1 tsp garlic
2 tsp onion
1 Tbl 5-spice
1 tsp sugar (or more if you prefer)
Triple or quadruple this, depending on how much you like to use.
I don't put salt in rubs. I salt the meat first--for butt, fairly generously--then allow the salt to draw moisture to the surface, 5-10 min. Then I apply the rub over the salt. The moisture makes it stick. Keeping the salt out of the rub means I can rub lightly, moderately or very heavily and not affect the salt level since that is applied separately. Consequently, I don't slather either--though I do do dry-over-paste rubs.
You could do an apple sauce slather if you wish but you'll need to tighten it up considerably first. To do this simply, mix 1 part mustard with 3 parts apple sauce in a saute pan and heat over med-low heat. Get it to a simmer then, stirring constantly, increase the heat to medium and simmer till some of the moisture evaporates and the mixture concentrates and becomes thicker and more pasty, just a few minutes. Scrape to a plate, cool, then use or store for use.
If you go the slather route, salt the meat lightly, apply the slather, salt the slather lightly, then apply the rub.