I would replace the two beams* supporting the rafters with five taller beams--four around the perimeter and one across the center, supported by the perimeter beams. The rafters are probably fine, assuming that they're continuous or split in the center only. This would allow you to eliminate the center post and the A-frames.
It's a bit much to go through the details of the entire project here, so I'll mostly list what you need. You can take it from there and post additional specific questions as needed.
You'll need ten 16 ft. 2x10 boards, all reasonably straight and with no giant knots. You'll need two double-2x10 joist hangers and suitable nails for them. You'll need 2-1/2" screws to sister the doubled beam members.
You'll need the understanding of how to connect the beam member pairs and corners well, and how to properly support them on your posts with suitable hardware. You'll need to know how to support the center beam with steel hangers.
Then you'll need to install the beams one at a time, temporarily supporting the rafters as you work to detach all the joist hangers and remove the undersized beams. You'll need a solid plan to accomplish this in a sensible sequence of steps. In short, you need an experienced carpenter on hand.
* The center structure isn't a proper beam. Its ends are essentially unsupported, making it a big cantilever balanced on the center post.
