Mistakes made

[lij45o6]

I don't play much SP21 but I thought you were only supposed to double down rescue with stiffs, aside from the sole exception of 17 v A.

The DDR rule(s) for Sp21 dictate surrendering a doubled 17 vs A.

https://wizardofodds.com/games/spanish-21/

[ZenMaster_Flash]

"The DDR rule(s) for Sp21 dictate surrendering a doubled 17 vs A."

Yes, the Basic Strategy dictates just that.
I had pointed out that at uncommon
True Counts, in either direction, Hitting or
Standing becomes optimal.

[Three]

The DDR rule(s) for Sp21 dictate surrendering a doubled 17 vs A.

It is deck composition dependent. But the vast majority of the time this is the correct play. High enough T density and you don't rescue. Of course you need an ace neutral count to know when that deck composition occurs.

[lij45o6]

It is deck composition dependent. But the vast majority of the time this is the correct play. High enough T density and you don't rescue.

In that case, with a deck rich in A-4 compared to 5-T, standing doubled 17 vs A is favoured, correct? Or would we surrender as the dealer has a better chance of drawing 18-21?

[Three]

In that case, with a deck rich in A-4 compared to 5-T, standing doubled 17 vs A is favoured, correct?

The 7, 8 and 9 are bad news as a hole card. The 6 is a push in S17. Any other hole cards bust when followed by 2 T's. Or a better way to look at it any hard stiff is busted by a T. If there is enough of a chance of developing the dealer hand into a hard stiff in a deck rich enough in T's the bust rate can be high enough to warrant waving off your DDR option.

So hopefully you can see the 5-T card grouping won't work because it is rich in cards that make a 2 card dealer hand and make too many hard stiffs into a made hand. The 5 makes every stiff into a made hand. The 6 makes a 12 thru 15. The 7 makes a 12 thru 14. etc. Ideally you want just T's against other cards. That means a level 2 system. Most (level 2) SP21 counts include the 9 as a (weak) high card just to get 1 extra low card into the count. The penalty is minimal since you pretty much always hit 12 against everything and never take insurance. The 9 as a hole card with the ace will make the dealer total 20. But you know the dealer doesn't have a T under. If he doesn't have a made 2 card hand the bust rate is high even including the 9 as a high card.

[Meistro123]

In that case, with a deck rich in A-4 compared to 5-T, standing doubled 17 vs A is favoured, correct?

No, quite the opposite.