In a loose–passive game that's deep, is it better to open-limp or min-open your entire playable range against a button straddle? What if the game is more tight–aggressive?

Open limping without a button straddle requires a deep, loose/passive and sticky table. A button straddle makes it worse for open limps and requires a button that never squeezes and has a very tight raising range, stacks that are still deep after adjusting for the straddle and the other players are still loose/passive/sticky. With a button straddle you would almost always be better off with a tight opening range and raise big.

I play a lot with this structure, I have no clue what the optimal strategy should look like, but I only open limp in the blinds if the straddle is at most 3x (preferably 2x) of what I've already put in and villains are mostly decent and aggressive. I have no calculation behind it, just a feeling that having a huge discount on a limp and getting into some very juicy limp reraise spots offsets the downsides of entering passively. I guess on a weak passive table you could also openlimp some faceup bottom-of-range stuff that is too weak to raise, but would take a flop cheap vs bad players.

Other than that on a nice soft table with little 3betting and lots of flatting I just 3-5x open (maybe more on dream tables). Here I think you just want to start building pots yourself with good hands because there is less chance of someone isoing and you having the chance to limp reraise. I wouldn't play a strategy that leads to playing most of my premium hands from the blinds in 6way limp pots.

FWIW most pros I play with do openlimp in the blinds, they mix raise and limp as I noticed, so it can't be too bad.

When I play SB against BB, I open for 3x with all playable hands to try to avoid paying rake. If the BB is super aggressive, I'll open–limp.

I've been min-opening against a BTN straddle. But I feel like I should take the same SB against BB approach.

I think I like tightening my opening range (maybe loosen up a bit in the BB) and opening for 3-3.5x against a BTN straddle to try to avoid paying rake. If the BTN straddle is super aggressive, I can open–limp those hands.

I think I also like being exploitable in a loose, passive game. Open–raise good hands. Open–limp small pairs, suited connectors, Kxs. (Is this the right approach in PLO?)

I actually hate games with a BTN straddle. It makes the game tighter, and people are less likely to straddle UTG. I also have to BTN straddle every time because it's slightly +EV—as long as the table isn't tight.

In the SB, you have the worst position at the table. With a button straddle, you lose the only advantage of being able to act next to last. You get less of a discount to call.

Play this like you would UTG but a little tighter.

I've learned through trial and error to into GG mode and limp my entire (very tight) range in the SB with a button straddle.

Almost all button straddlers will raise wider than they normally do overall. So, you can limp/3-bet the straddle and pick up nice change without a flop. Limp/3-betting straddle pots is very profitable in $1/2 games. You get to evaluate the strength of everyone else's hand before deciding whether or not to 3-bet. A lot of people will complete from the small blind with a wide range on a button straddle, so limps are very disguised and natural looking.

Raising telegraphs that you have a huge hand since position is the worst possible on all streets. A $12 (6x BB) raise in a $1/2 game might get 4 callers, but a $25 (5x BB) raise in a straddle pot rarely gets called in my typical games. So, it's either raise and take the $5 straddle or limp with a disguised hand and play poker.

Edit: not enough info in OP

Number of limpers is material to our raising strategy.

And if this is a mississipi BTN straddle requiring sb to act first? OP should edited to clarify

Yes. Are there poker rooms where first action isn't directly left of the straddle?

Zero limpers. We're the first one in. See title.

I'm not sure how we started focusing specifically on the SB—probably my fault for not being clear. But my original question was what was optimal as the first one in from any position—limp, open 2x (half pot), open 3–3.5x (full pot)?

If everyone folds to me in the CO, should we also limp our entire playable range (which is tightened up a bit—as if we were playing from the HJ)?

Yes. GNLV, action starts on normal Utg then when gets around to button, button skipped. SB, BB act then we go back to btn.

This btn straddle is a version I almost like.

Btw @earlier post… MS straddle and button straddle are different.