Loose skin after BBL. What can be done to improve this? (Photo)

Based off your photos (and no before pictures to compare it to) - your abdominal skin and back skin will have a period of looseness post liposuction from the fat being removed and the skin needing time to retract or contract. The problem is, this doesn't always happen, which results in loose appearing skin. If you continue to decrease your weight and tone (esp the back and the core areas) that can help with the skin tightening some.
There are a few skin tightening modalities that can help potentially get some more skin tightening before you just give into a abdominoplasty -
1. Internal energy - something like, J Plasma (needs to be done with liposuction) helps contract skin and procedure more collagen while you heal post procedure over the following 8-12 months (problem here is that you've had lipo once already, it works less each time). 2. External energy - Morpheus is an example - applied to the external skin everywhere you want to be tighter - uses radiofrequency, heat, and pins to help tighten skin and also stimulates the skin to produce more collagen. 3. Decreasing your body fat with weight loss and muscle building - also helps with tightening the skin more.4. You can use a combination of these. 5. Cutting the skin - aka abdominoplasty.
The flat parts of your butt (hard to see on the photos and no before photos to compare) - are likely some spots that the fat didn't survive or areas where you put pressure (to sit, to sleep). So can try to put fat there again and see if it improves - which would require fat grafting again.

The flattening in the lower part of the buttocks, especially near the vertical gluteal line, is caused by a very dense and short septal network in the superficial fat layer of that area. Due to this septal network, it is very difficult to add fat to that region, and flattening is inevitable afterward. The way to overcome this issue is to release the septal network using subcision or a V dissector. This will correct the flattening caused by the tight connective tissue bands.

I would suggest having a second BBL. It will improve your shape and even out the areas of liposuction and fill in the divet in your buttock. I don't see how it would be possible to perform a tummy tuck on you, to me you don't have excess skin to allow for the performance of a tummy tuck.

Usually you have to have had a significant weight loss or children in order to be a candidate for a tummy tuck.