I think you help raise some questions about usage that can cause us to stop and think about the interesting phenomenon of us homeowners trying to "own" so many possible experiences in our personal places of residence--the cocooning phenomenon and the idea of having a spa-like envrionment at home. Sitting by a firepit seems, or is, nice on an occasion when we have experienced it, so maybe we should have access all the time. Sitting in a hot tub is nice, so get one of those. Watching koi in a pond is nice, so maybe have that. And so on. I'm not at all trying to say that you are on the wrong track--instead, am agreeing that you're doing a good thing to think, how much will I use this; will I end up forcing the "let's sit out by the firepit 'cause we have it and need to make use of it" when you'd rather curl up with a book in the house; as you note with the question of wood vs. gas, do you anticipate having several hours of continuous use of the pit once you've got the wood going, or are you more likely to say, well, my hour is up, got other things to do. Weather and climate are huge issues too, since I can clearly see some climates where it is cool and delightful (with no mosquitoes) and firepits or outdoor fireplaces really could get a lot of use; you'll need to decide if it's for "show" when weather is really mild (and will it bother you to burn wood), or if weather is kinda nippy, will you really want to huddle round the pit; will everyone smell like woodsmoke, and will they like that; do you have kids like teenagers who might in fact "hang out" at your house if they can burn something, and that could be a good thing. The conversion to table or usable floorspace is a neat idea and might sway you, since it'd be there if you want, but not the gaping pit; OTOH, that pit, raised or flat, still won't be as flexible for as many uses covered as would plain patio, if you're limited on your total space and "floorplan". One of the things I realized was that I just didn't currently live the life of the firepit or some other similar creations, and I ought to instead take trips a few times a year to experience some of these things in a vacation mode, but not try to own them for infrequent in-home experiences. But still it makes perfect sense to try to identify those things that you will really use, and more conveniently and less expensively, by having them at home.

Depending upon which code you are under, copper may not be acceptable. If using black pipe, it ought to be coated AND taped to prevent corrosion. It's a poor choice regardless, particularly under a slab where pockets of gas may be trapped an explode. And don't forget your tracer wire. You may also want to check the legality of running this pipe yourself - in many areas, unless you are specifically licensed for this type of work, you may not do it yourself. At the very least, have the work inspected - a leaking gas line under a slab is a serious matter. There is a reason the estimate was high.

Does the pit have a cover that can be used during a fire? Is the deck attached to the house, or close enough to it if on fire would pass to the house? My fire pit can shoot embers out up to 6 ft in either direction in certain conditions. Personally, I think its too risky for me and would assume it would never pass code. You might consider modifying or replacing it with a propane or natural gas fire which would be safer.

I think it looks a little funny for a few reasons. 1) it isn't big enough for your space. A bigger square would work better there. and 2) In my opinion, it is a bit too tall with your low sofa. I know that they make lower lying fire pit tables where the propane tanks lay on their sides (still totally hidden from view).