I found this link for a 2016 Lexus IS300 alternator. While capable of up to 240 amps at higher rpms, the output seems to be less than 50 amps at 750 rpm.
https://www.tucsonalternator.com/product/1006264/2016-Lexus-IS300-Base-3.5L-V6-Alternator/0-GB/
Me, I’d probably just drive the car more often or recharge the battery once in a while using my shop battery charger. And turn the car on and off when I wanted to. Then monitor the situation. In any event, you’ll almost certainly get better results at a Lexus dealership than a retail parts store. BTW, I was recharging my Corolla’s battery using a shop battery charger just this morning, common routine maintenance for a car not driven frequently. I like to keep the Corolla’s battery fully charged b/c it reduces wear and tear on the starter motor.