Fuel for thought.
DI is great, however it has limitations. The deeper the lake the bigger the cone. If the lake is shallow, you wont get much information and could actually miss something next to the boat. Literally next to the boat, you might see it with the naked eye, however the sonar unit wont be able to see it unless you drive right over it, and at that point you may have scared off the fish.

SI is great to find fish off to the side and narrow your fish area. However if you mount SI to your trolling motor, what your seeing on the right side of the screen could be anywhere. All depends on which way the trolling motor is pointed. If it was pointed straight ahead (12 o'clock) the right side of the screen is the right side of the boat. Spin that trolling motor 180 degrees and the right side of the sonar will display what's on the left side of the boat. You could spend all day chasing an image/cover/structure.

Personally, SI at the council and DI/2D on the trolling motor.

As for mapping, I have an 899 at my tiller council and use the transom transducer for mapping. I use autochart pro and have to create my maps at home on my pc. I could hook up my pc in the boat, but it isn't water proof, and doesn't really gain anything. While making my maps from my Lazy-Boy I can take my time and scan the map for unknown spots. Something I can't do while driving around the lake making the map. While gathering data I need to pay attention to where I'm going, who else is on the lake and where they are going, etc. Basically I don't have time to watch the sonar screen for new spots. I'm paying attention to creating the map, not reading it.
Plus from my recliner I can replay the SI data and find unknown cover (trees, brush piles, stake beds, etc.) and place my X on the spot to check out later.

FYI, mapping a lake takes time, lots of time. With my AC-Pro I can cruise 4-6mph. Seems fast, however your going back and forth dozens of times to get bottom detail.
Deeper the lake, the wider the data swath collected. Along the edge of the lake, in say 15' of water your SI will map out 50' or less on each side. In 30' your out to 100' on each side. 50' + and you could be out to 150' on each side. Plan accordingly. It is a challenge to go back and get a spot you missed. You have to know what the lake level was the day you went, and the day your going out to record the missed spots. you can compensate with offsets while making the map, but if you don't have that info your map will be off in those spots. Start by mapping the places you fish the most, map to and from them and pick a different spot fo map each time you go out. If you can dedicate a couple hours per trip it won't take tooooooooo long.
I have about 24hours of mapping on a 900 acre lake and still have about 1/2 of it too map. Mapping take time. But, if your lake isn't on one of the map cards, its the only way you will greatly improve your fishing on that lake. Plus the bonus of reviewing the SI data to find cover. I have trees marked in deep water no one else knows about.