This is a really important topic. I believe that the blinds represent negative win-rates for most players, especially if you don’t have a huge edge over the field. I think even high-stakes real-money cash players are fairly happy to just about break even from the blinds (is that true?).
I believe the broadest optimal strategy that would help the most players is to defend the BB more and complete from the SB less.
First the SB: people complete way too much and way too wide. You are going to be playing out of position against the entire table and don’t even close the action. In poker in general you should always have a plan for how you are going to win chips in a given situation, and if you ask yourself “how am I going to win here” most times from the SB, the answer is that you aren’t. The best case scenario where you flop a monster, it is highly unlikely to be the nuts with the kinds of hands that complete and you will be first to act in a multiway pot out of position.
The pure equity math for completing may be fantastic, but it is extremely hard to realize that equity. Here’s an example, you get Ts7s and complete against 3 limpers (and BB checks). The flop comes AsTh7h, which is a fantastic flop for you. But what do you do? You have a tremendous opportunity to get big value from Ax (which limpers may or may not have a lot of), but any hands that continue against your bet are going to have tremendous equity. Ax hands probably have at least 5 outs, flush draws have 9 outs, and 98 has 8 outs. Plus there can be A7/TT/77, which have you just about dead. My point being that you have a relatively strong hand where you would want to play a huge pot, one of your best case scenarios, and it’s still very dicey in a multi-way situation out of position. If you flop top two pair (like T76) there aren’t many hands you can get value from and you are still vulnerable. Basically, any hand that completes from the SB is going to hate 95% of boards (even the ones you hit like flopping top pair kings with K9o or flopping an open ender). Even if you flop the stone nuts you are in a weird spot to get value from a relatively small pot where you often block some 2nd best hands that continue, so if you bet large the table might fold around and if you try to slow play the table might check around.
My advice from the SB would be to be more selective about which hands complete (like weak pocket pairs or suited connectors can be ok because they either hit the flop hard or are easy to fold), but to do more raising with hands that are ahead of your opponents’ limping range (like ATs, 99, or KQ).
When facing actual opens ahead of you, you want to be pretty selective about calling. You are likely going to price the big blind into a call and be stuck in the middle. The 0.5 bb you have invested doesn’t really help your pot odds much against a 3.5x+ open size, so it can be better to let marginal hands go, especially if you opponent is likely to be opening QQ+ only. On the other hand, the strength of most players’ opening ranges means that there are more implied odds for you. But in general, I think it is better to 3-bet or fold almost always from the SB rather than make a “standard” complete. I fold some pretty tempting trash hands from the SB rather than complete against a bunch of limpers because correctly calling postflop when I flop middle pair or top pair with hands like Q9o/87o/95s is going to cost me more chips than it is going to win. In most situations on Replay those are not the kinds of hands or spots that will be the most profitable, and there are plenty of better situations to focus on.
From the BB you are getting better pot odds than the SB (you have already invested twice as much!) and you may be closing the action. That makes defending the big blind a better proposition than the SB. As previously stated, many players have extremely strong open ranges, but the implied odds are good and you are usually priced in, especially with other callers ahead of you. But you still need to ask yourself how you are going to make the spot profitable and you can end up in similarly dicey postflop spots.
As far as when to raise from SB or BB over limpers, I think they are fairly similar, though I generally prefer being in the BB to the SB. Players on Replay tend to be calling stations, and the way you punish them is by being value heavy. You don’t get there by opening 76s from the blinds over a few limpers. Don’t get me wrong, you CAN open those hands, and maybe you should if you are playing good opponents who force you to balance your range. But in general people will call too much and you don’t want to raise hands that are going to force you to semi-bluff out of position a lot of the time postflop when you could just see a free (or cheap) flop. Again, in a strong game against good opponents I would definitely mix those in because they are well disguised and also disguise the stronger hands in your range, but a lot of the time here you just don’t need to do that. People will call your strong hands anyway, so you can get value, and people will call your weaker hands too, which can lose you value. So, you want to raise the kinds of hands that dominate limped hands, like strong broadways or largish pocket pairs. Just my two (or more than two) cents on the topic.