You can tech chase off bthrow, I generally prefer dthrow, unless my bthrow puts them in the corner, since that makes reacting way easier. Or I guess if you dthrow and they go into the corner you could bthrow back to the stage to keep it going.
General tips for dthrow tech-chase:
1. DI away->walk forward to follow the DI
--tech away->dash->full wd->grab. Hardest one to do on reaction.
--tech in place->standing grab
--tech behind->dash back->grab
2. DI in->turn around in place
--tech behind (your current position)->wd back->grab. On falco you may need to dash grab
--tech in place->standing grab
--tech away->wd forward/dash grab
Biggest things to account for are falcos long ass tech away and shines/buffer options out of the techs.
Falcon is by far the easiest to do on reaction.
You can also CG him on reaction, but it's harder than tech chasing.
As far as marth goes, it's really hard as sopo.
Marths like to dtilt you a lot to stop you from approaching on the ground. Easiest way to beat that is just to sh fair if you know when he'll dtilt. Be careful of fair/fsmash mixups to counter that.
He sometimes also tries to bait if then DD grab, you can spotdodge or maybe get out a jab/ftilt if he's slow.
Marth shouldn't really be throwing out fsmashes in neutral, but if you figure out how they are trying to fsmash you can wd oos punish it. I wouldn't rely on them throwing out dumb fsmashes though.
Marth has a gimmicky bthrow->tipper fsmash combo on you at low-mid percent if you DI in, they usually do it near the ledge. I like to get hit by that sometimes. DI down+away avoids it, but you're still in a bad spot.
Marths aerials are unsafe on shield, if you know marth will jump, you can wd into him with shield up to force him to misspace and you can usually get a grab.
With fair marth can do the following:
1. Double jump away: wd forward->uair/usmash
2. Double fair/double aerial: if misspaced, hold shield->grab. If spaced probably get out of there.
3. Uuuh this one is weird. No marths really do this, but while he can't just fair->Waveland, he can do late-ish fair->dj->airdodge on first frame of dj in order to effectively do fair wavelands.
Fair waveland back->dtilt/dd is really dumb and super safe and I don't know if sopo (or ICs in general) actually have a good response to it.
But no marths really do that yet so we don't have to worry just yet.
Uuuuuh as far as combo game goes most of your conversions come off of grabs and fairs as sopo.
Grabs generally you just dthrow->aerial or dthrow->usmash.
On full DI away you actually have a dthrow CG. I forget how high it goes off the top of my head, but it lasts a decent amount of time. Slight DIs not so much though, at low percents I like dthrow->nair->regrab on things that look like DI away.
DI in at low percents you can usmash, then at higher percents you can uair or fair and get something started off that.
A lot of times your opening is fairing marth on the ground (out of his dtilt). Usually when this happens he asdis down (because he's dtilting) which forces a knockdown rather than popping him up.
This is good because you can either jab reset/hit him and/or react to his get up options. Generally that's really easy to convert to a grab/smash attack
So yeah.. That's really all I can think of right now as far as general advice goes
