My proposal for our next project would be to pick the heavy cruiser, and essentially make the Miranda or Nebula to the Ambassador's Constitution or Galaxy class. Stick entirely with existing technology, and produce a capable workhorse heavy cruiser integrating the R&D from our flagship explorer which can do well at almost any mission and be built in volume. We should re-use as much as possible from the Ambassador design because the largest component of cost in a project like this is the cost of learning, and we've learnt those lessons now.
That being said, there's nothing wrong with the fact that the Ambassador had a ton of prototype technologies and was extremely expensive to produce. This is something we absolutely knew and were prepared for going in, and has produced an absolutely stellar ship which has pushed forward the frontiers of starship design and won immense diplomatic plaudits by demonstrating the spirit of Starfleet to other peoples.