P.S.
I actually still have my SE3 on dual SIM right now with my Philippine eSIM on roaming (need it for OTP for Philippine bank accounts and e-wallet). Battery life seems the same as when I only had one SIM.
I'm using an eSIM and physical at the same time on my 12 mini, and to be honest, I can't tell the difference in battery drain even though this mini of mine is now going on 5 years old, so it is starting to show its age either way. The mini range has also not had the best battery life to begin with which is probably why Apple killed them off.
But right now, if I'm at home doing nothing with my phone and even if both sims are on, it will stand by for the whole day. Yet when I go out and even with only 1 sim in use I can kill it in a few short hours; streaming music via BT headphones, taking photos and video, and of course the biggest battery killer; navigating with GPS.
Anyway, I've got my eyes on a 15 now (new), where I'm guessing dual sim is a non-issue when it comes to battery drain rate, even if signal is weak. What I think really matters is
what you are doing with your phone, not number of sims in use.
The reason I'm looking into using dual sim on a daily basis for maybe a few more months; a new provider in Belgium has just come to market with a €5 plan that gives you unlimited local calling, €0,01 calls to the rest of Europe and 15GB data, all of which you can use in the rest of the EU. But I want to test them out a bit first with my primary number still with a well established provider. We have these functions on devices we pay good money for, so might as well use them.