short 2x (or 3x) short stock

[1nvest]

Half in 2x long stock, half in bonds, rebalanced yearly tends to track 100% 1x reasonably well. A variant of that might be to 25% 2x long stock, 25% short of the 2x short stock which can be more defensive in times when there's high downside volatility such as in 2008 where in losing less you subsequently see higher overall gains (13.5% annualised instead of 10% annualised since 2007 in that example).

Any suggestions as to brokers/platforms that support that however, i.e. primarily short SDS (the 2x short S&P500 type ETF)?

igindex with whom I had a account many years ago don't cater for shorting SDS

More specifically, after a pretty good up-run since 2009 it feels like a reasonable time to move to more defensive and hence my inclination towards 3USL and 3USS 3x long/short pairs with a 16.6% allocation to each in a similar vein to this

So far I suspect TradeStation might, but not sure.

[1nvest]

With the recent strong gains driven by tech, maybe a min dot com type 2000 bubble burst could be in the pipeline, in which case something like this yielded a 6% higher annualised since 1999 (13.8% annualised versus 7.5% for the S&P500).

[NotSure]

...Any suggestions as to brokers/platforms that support that however, i.e. primarily short SDS (the 2x short S&P500 type ETF)?

Maybe try asking here: https://www.reddit.com/r/wallstreetbets/

Trading 212 have a wide assortment of such instruments, but not sure they have the specific ones you are looking for.
https://www.trading212.com/en/Trade-Equities