Scrapper is perfectly fine for bussing
Scrapper is perfect to buss. In terms of builds, I'm not sure why people give "Adrenaline not good on Tai" advice , but it is not true. I'd say it's not necessary to set up a pre-relic and it's a bit excessive in the event you go for a full critique.
For 1370 dollars for a 3-x3 you can choose to go Tai/Grudge/KBW. But it is possible to throw in substitute Ambush Master or Adrenaline into the mix as well. A majority of combos perform perfectly. There are many who go 50/50 between crit and swift Some go for full speed You can experiment with the ratio a little in Trixion and decide which you like the most and they'll be able to perform the same.
As for a 4x3, I would recommend Tai/Grudge/KBW/Adren, and again, Ambush Master can also be substituted in. If you're interested in going 4x3 and 1/2, Adrenaline could be dropped down to 1/2.
Your 5x3 is based on the stat spread and the relic set you choose. Entropy is the attack that has a high damage ceiling, while Hallucination is a consistent non-positional, but lower damage. Personally, I'd recommend Entropy due to the fact that the amount of time needed to defeat Hallu isn't a lot in any way. The majority of players will choose Entropy and that's a major reason for why Adrenaline is a great choice for Scrapper. If you're playing KBW on Entropy it is essential to have near to 80-85% crit for it to be effective, because you're stacking crit damage.
If your stat spread is closer to 50/50, you can go Tai/Grudge/KBW/Ambush/Adren. If you're playing at full speed on Entropy it is possible to swap KBW with Raid Captain instead , since you'll aren't as efficient with KBW and have less crit. I hope this helps!
Edit: I was thinking you were playing Tai, however Shock is also fantastic it's just an issue of preference in playing style. Shock has full-spec, with crits on necklaces and another accessory. Engravings are the same, Ambush maybe taking higher priority since back attacks are much more vital as well. You also are able to swap KBW in exchange for Cursed Doll if you want to add it to your 5x3.
I'd only suggest Adrenaline in a full swift Taijutsu build, If you're doing 60/40 or 70/30 crit/swift , you'll be in the middle of:
45-49% (base crit) + 17 percent (entropy set) + 7 % LWC (card created) + 10 10% (back attack) and you'll have at or near 80% crit, this is where KBW is at around 20 percentage more damage.
When you consider the synergy of crits from parties, which is a range of 10 to 18% crit that means you won't overcap in crit using this method. If you go to Adrenaline 3, you'll be beyond 100% crit which is a waste of numbers.
I choose Cursed Doll rather than Adrenaline as many of the famous KR Tai Scrappers on Loawa go with: Tai 3, Grudge 3, Cursed Doll 3, KBW 3, Ambush 3, Adrenaline 1 & 2 using a 50/50 rapid/crit set-up.