It seems like you will need some on-site professional advice. I'm no where near that, so can just add some reactions. Your home appears large and a large financial investment, so you want to keep things appropriate for the setting and not tacked on here and there. The low level by the basement is not very easy to "connect" well with the high deck so as to have the 2 spaces function really well together. The other side yard that is a higher elevation seems the most natural area for spillover from a large gathering and I would be tempted to develop that space preferentially, even if it requires some privacy screening from the front view of the property. However, if the basement room--whatever you will be making that in to--is a key action zone that will be highly related to outdoor activities, then maybe you do want to develop that walkout area more. A hot tub under that deck doesn't seem very appealing. So, to come up with the best design, you may need to flesh out your activities and traffic flow a bit more to see what is most rational from that standpoint and then factor in the $$ cost of various solutions. Also prioritize--you are trying to do this--decide if pleasant seating out of the rain is most impt, or heating in cool but clear weather, or being in hot tub, etc. I'm not sure you can build a fireplace on the deck proper, if that's what you meant.

Just get straight glyphosate - it doesn't have to be RoundUp specifically and there are less expensive brands of this herbicide on the market (Ferti-lome sells a product called Killzall that is less than half the price of the equivalent concentration and size of RoundUp). If you do go with the RoundUp, just select the Weed and Grass Killer - no need for any other additions to the mix. As I noted previously, you may very well need several applications and it is suggested to allow at least 2 weeks after the last treatment before reseeding. And it is too early to reseed - too hot and too dry for most northern, cool season grasses to germinate well. Generally, the ideal time for fall seeding is early fall - September to early October, depending on your specific climate. And any cool season turf grass blend recommended for your area is fine. Most independent garden centers - not the box stores - will have blends that are appropriate for your area.

The runs for the septic start two feet out from where the gravel ends/lawn starts at the end of table. The runs for the septic are parallel to the house running to right when looking at the picture. They run every two feet covering the rest of the back yard. We thought of a raised deck at height of patio doors but really like being lower in the yard or a combination with dining area upper level down to BBQ area. We like to BBQ in the winter and since we have lots of snow, having the BBQ area closer to the door of garage made more sense and would be easier to access keeping the snow and water fr9m tracking into the house.

Transferring the same scheme to the plan, adding a little more. (There could be patches of flowering perennials in the groundcover bed and a couple of tall shrubs at the sides of lot for privacy. You need to refine the plan, working out details. Or work out a new scheme that better suits your goals. Mainly what I'm shooting for with this scheme is creating privacy and a view backdrop to enjoy from the patio. The foreground would be relatively level turf.