I'd be inclined to keep the Simplex shifters, if those are the retrofriction types. I've used a similar design Shimano SL-422 Light Action downtube shifter on my old steel road bike and liked 'em. There's an internal spring setup that appears to be very similar to the exploded diagrams for other retrofriction shifters. It was quite smooth and easy to use, compared with other simple friction shifters -- although friction shifters with micro-click detents are pretty good too.
I've since switched back to the original Suntour GPX Accushift setup, but the friction shifters were more versatile for switching between wheelsets. I use one rear wheel for the indoor trainer, and another for outdoor rides. There's a slight difference in tolerances that hinders crisp shifting on the wheel I use on the indoor trainer, but I don't bother adjusting it since I don't shift much on the trainer.
I also have older Dura Ace indexed downtube shifters, and I think I may have a set from a 600 Tricolor/early Ultegra group. And a couple of Shimano bar-end index shifters, one 8-speed Ultegra, the other 10-speed Dura Ace from a time trial/tri-bike with aero bars. I'm currently using the Shimano 8-speed bar ends on my hybrid with albatross swept bars. Excellent for that use.
A few local friends use bar end shifters on their drop bar road bikes. I'm considering that for one bike but I'll need another set of handlebars. Their bikes are set up for casual riding, with wide drop bars.
One problem I can foresee with bar-end shifters is striking the bar-ends with my knees. I'm tall and skinny and prefer fairly narrow 38cm or 40cm drop bars. However when I stand to pedal up climbs or sprints, my left knee occasionally strikes the bar-end mirror. No big deal, the mirror is on a ball joint so it just moves out of the way and I re-adjust it after sitting to pedal. But that would be disastrous with bar end shifters on climbs or sprints.
So if I decide to try bar end shifters on a drop bar road bike I'll need wider bars, or at least bars flared to 42cm or wider in the drops. Some rando and gravel bike bars would probably do. I could try cutting the ends from one of my current sets of 40cm or 42cm drops and see if it prevents knee strike without compromising handling feel. I also use the drops when standing to climb or sprint, so I'm picky about bike balance and leverage in the drops.