Twashtar is excellent in and out of abyssea. You'll notice it has similar DMG/Delay to Mandau (+dex - attack -1 base DMG). The weaponskill is similar to Mercy Stroke (flat out better pre-90 mandau, then it's similar @ 100tp). If you want a good dagger, get a Twashtar. Though since you are planning to play more casual, I'd recommend picking up a Rapidus Sax (100% drop), and a Triplus dagger (NM is killed all the time, you might be able to bum one off a group farming empyreans).
You can wear 4/5 af3+2, twilight, and any pair of +3% haste boots to cap haste.
I don't know what's better between triplus/sax and twilight/sax, they're pretty similar.
If I was 75, like you, I'd just focus on reaching 90 first, then worry about gear. Farm the raiders+1/+2 set, get a twilight belt, Aias bonnet, loki's kaftan, atheling mantle, and appropriate atma, and your thf will be pretty set (+ other standard ws gear you might already have, like hecatomb, dex rings, gorget/Lovetorque etc, I'm trying to think of abyssea gear you wouldn't have as a newly returned player).
after reading SES's thread I realize that there isnt so much of an emphasis on dps or trying to deal massive amounts of damage. So forgive me, I feel rather lost. Before, I always tried to play at the best of my ability. I got the some of the best gear that I could reach at that time. And THF was considered a trash DD,
Gearing yourself as well as you can still pays off, despite the lesser focus on all-out damage from days past. The only difference is that gear has much less of an impact than it used to, for now, as inside abyssea Atma enhancements dwarf most of the stats you get from gear. If the main endgame content leaves abyssea, gear will become more important again.
what is there to do besides just sit and grind for gear
That is what endgame has always been: Grinding gear. Personally, I think it's much more enjoyable now, than it ever was.