The best steroids for keepable gains will be the mildest compounds or ones that increase strength via lean tissue accrual and not temporary leverage increases or neurological supercharging. All steroids generally drive lean tissue, increase water retention, and upregulate nervous system output, but they tend to do so to different degrees.
Tbol and Dbol are very similar in lean mass gains for example, but Dbol puts on a lot of bloat that adds extra bodyweight (which increases intertia) and extra leverage (thicker bodyparts mean tissue binds up around joints sooner & stores more kinetic energy).
Halotestin adds virtually no lean mass but causes a massive temporary surge in neurology. You take it & immediately get stronger, you stop and it’s all gone.
Deca isn’t thought of as a strength drug much, but if you hold 10lbs of water on it that leverage will be gone post cycle.
I would look at modest doses of Var, Masteron, Primo, Eq and a low dose of testosterone with any of them. The more you take, the more you should expect to lose post cycle, so think fairly low dose.
I haven’t seen enough return with SARMs to really recommend them for strength, BUT if you saw those kind of results with Ostarine then considering a run of LGD does make sense to me. Rad140 is a great SARM for strength, so don’t use that if you want lasting gains. LGD is better for muscle growth with a decent side effect profile.
TLDR: You basically need to grow new muscle tissue, without a bunch of water weight during the training cycle that will temporarily drive strength even higher but be gone post cycle. I know you don’t care about size, but that’s the only adaptation from steroids that can remain in their absence.