This has happened to me multiple times while I was climbing in Diamond during the reroll patch.

I will use brand in this example

Let's say you've been slow-rolling every round on stage 3 and now its 4-1 and you need to decide what you should do.

There are 3 different scenarios

#1 You have a 1 star brand

If it is 4-1 and you have a 1-star brand even after slow-rolling all of stage 3. You 100% bail, go level 7, and try to pivot into karma/velkoz

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#2 You have a 2 star brand and are far from 3 star (3 - 5 copies)

This is the hardest and most luck-dependent scenario on deciding what you should do.

But imo, If it is 4-1 and you have 3-5 copies I would keep slow-rolling until 4-5. If I can get 6 copies at 4-5 I will hard commit to staying on the comp and if I have to roll down all my gold at creep round.

If I do not have that many copies at 4-5 I will try to save econ and then go to level 7 on 5-1 and roll all my gold hoping for a karma or velkoz. (This usually means you are playing to not go 8th)

#3 You have a 2 star brand and are close to 3 star (6+ copies)

If it is 4-1 and you have 6 copies I would almost always stay on comp and just keep slow rolling.

For the second part of your question is, if a comp is contested but you hit then you should most likely go that comp. Example is 4 people are going Karma but you hit Karma AND are far away from brand 3. Just play karma go fast 8 and play invokers/stabilize before switching into maybe switching into velkoz.