You need to define small... do you mean small budget or small SKU count or both?

If the ad account is already running then smart bidding can work really well. When you switch comes down to having enough data and being in the right season (peak season) vs just picking a random timeframe.

We generally break out DSA in a campaign to act as a sweeper to mine for new keywords we didn't know we should bid on. If you are thinking of doing SKAG for search campaigns, makes little sense for smaller brands as they rarely have the search volume to make the management worth it. Most brands should do single theme ad groups (STAG) to get data and scale.

How you use standard shopping vs PMax comes down to control. May make more sense to put profitable products in standard shopping to gain more control and target certain keywords.

I would do remarketing across search, shopping, DSA, and discovery ads before I do remarketing on YouTube and Display.

I would build new campaigns based on what has performed well in the past.

When you're done, switch on and pause all the old stuff. Deleting is never a good idea as you won't be able to review performance data. If you want, you can change the name of all the old campaigns by placing something like "Z" in front. This way when you view "all" campaigns your new ones will always appear above the old ones.