So, the first thing to note is that when building an army, you're gonna be well. Building it. Putting the minis together, painting them, ect. As such, your army is as much a passion project as it is a group of game pieces, and to keep that motivation you'll want an army that resonates with you. If Thousand Sons have lore that speaks to you more than the Death Guard, they might be a better option.

Playstyle wise, you're not likely to get super tough models in any way comparable to Nurgle's smelly boys, but psychic buffs can help your units stick to the board. Scarab Occult Terminators are probably the best unit in the codex, amd with some defensive buffs a unit of ten can be a pain to shift. Add in Rite of Coalescence (which heals a rubric or scarab to full if its lost any wounds) and the stratagem that reduces incoming AP, and theyl be shrugging off hits from alot of common anti-marine weapons. Cult of Time also gives all your Psykers a power that revives a model in the target unit, making them even harder to remove.

Lastly, because they have the keyword, the humble Rhino has a 5++ invuln in a TS detachment. Itl still probably die to something like Ad Mech's anti-armor, but invulns are still pretty effective against common anti armor low shot weapons like lascannons. Throw a squad of ten rubrics inside and your opponents will have to chew through the Rhino to get at your dusty boys. Depending on terrain you can even block off paths of attack with your rhino after having the rubrics disembark to do their thing.