Sorry to respond so late, I haven’t been on the board lately.
DO NOT carriage bolt your beams to the posts, rather, set them on TOP of the posts using Simpson Strongtie T brackets. If you only bolt the beams to the posts, your bearing capacity is determined by the “bearing area” of the support, which in the case of the bolts is ONLY the 3/8” by 6 or so inches long. By setting your beams on TOP of the posts, your bearing area is the entire area of the post, or 3.5 x 3.5 inches. To “splice” your beams, using 2x10s (no smaller,) alternate the seams so that on one side, your 2x10s will be 13.5” long each, or in other words, just two pieces with a joint in the middle, situated directly on top of a post. On the other side, you would have a 2x10 which is about 6’9” long, a 13.5’ long, and another 6’9” long piece, or in other words, three pieces with joints at the end quarters, again situated directly over the posts. You will need to separate the two boards with a sheet of exterior grade plywood, 7/16” thick (they don’t make ˝”) to allow the “beam” to be the same width as your posts. Use Liquid Nails and a few carriage bolts to “suck” the beam parts together.
As far as your joists, I recommend 12” oc joist spacing, which only adds $10-20 to the cost of the deck, but does wonders for strength and stability. (I hate bouncy decks!) Also, if you want your deck really flat, you should check ALL of your joists for uniform measurements. Pressure treated 2x8’s (no narrower, 10’s would be fine too, even better!) tend to vary in width, so your deck may look more like rolling plains, than a flat plane. To compensate for this, rip all of the joists on a table saw or radial arm saw to the width of the narrowest board, and mount the joists cut side down.
Use pressure treated lumber throughout. Your posts will last longer and work better if you set them on concrete pilings set at least 3’ into the ground, topped with a 3/8” J bolt, then secure the post to the bolt with a Simpson adjustable post bracket. You can line up the J bolts using contractor’s string and a few stakes.
Decks are easy after you build a few, but the first couple are a pain to learn. Read LOTS of books. Sunset and Ortho are good, Home Depot offers lots of information as well. Feel free to ask more questions directly if you like.