Based on your Marriott loyalty & need for a mall-attached property, may I suggest Le Royal Meridien attached to Vendome? Definitely lots of room to exercise/walk in the mall, however note the mall connection is closed midnight - 5am. Only downside is there is no lounge by virtue of elite status, only if you pay for a club room. However, on the flip side, I've been upgraded to the residences side every time which has given me a full kitchen, living room, and bedroom. The breakfast is also good enough to hold you until dinner for sure (I've stayed up to 5 days without having been able to eat once from every dish). Lusail as an area is up-and-coming, with many government offices slated to move there, but outside of the many hotels nearby there will not be a lot of independent dining options, although within Vendome there are plenty of food options. Outside of Souq/Jasra, you won't find too many restaurants that aren't in malls/hotels.

Grand Hyatt has everything else you're after (including a pretty varied lounge and a great host team), but it's about a 10 minute walk across a multi-lane "highway" to Lagoona, which is quite a dead mall & quite a maze to navigate if you're unfamiliar with the layout.

MOQ has a hotel attached, however it's not an international chain and I believe also dry. The mall is also on its last legs with nothing nearby. Villaggio has The Torch attached however it's not a hotel I'd like to stay longer than 24 hours in, much less your extended stay.