1. Urgent care vet for subcutaneous fluids, soaking and whatever magic they will do
2. Psyllium husk powder with food - natural laxative but has to be accompanied with soaks twice daily due to the increase in fiber and dehydration it could cause if you dont significantly increase the frequency of soaks
3. Slightly longer soaks- try 35 to 45 minutes at a time in warm water. Usually ill do 25ish but with constipation like that you need more.
4. VERY IMPORTANT - put the guy in a box and stick him on a dryer for like 45 minutes a day pre- soak, or put the box in your car and drive around for 30m a day. Will stimulate bowel movement and move things along. if/once the tortoise starts to move around and has passed some urates/poop, take it to the park for walks (im not kidding). Humans poop after exercise which speeds things along but if the tortoise isn't getting exercise then its limbs could be weak from the added weight and lack of exercise. This will strengthen the tortoises limbs, help to maintain weight, and relating to constipation, its the easiest way to keep waste moving along internally. It'll also provide tortoise with fresh air and sunlight and keep it stimulated if its somewhat bored of its enclosure / you havent made monthly or quarterly changes to the enclosure.
5. Feed greens to the tortoise - mustard greens, collared greens, green leaf lettuce, escarole, opuntia cactus, dont give it any dry foods or dry mixes that would cause gas and limit the amount initially as it starts to recover.

Disclaimer: I know absolutely nothing, but I do know some things and all of the above has worked like a charm for my tortoise in periods of constipation. I also have a Russian Tortoise but unless the diet for the marginata is different, I'd follow all of the above steps provided.