Considering that even the hybrid cars made by car manufacturers themselves are heavier than their ICE counterparts, I don't think that by retrofitting an older car you'll get a better result.
For example, 2024 Hyundai Tucson curb weights:
Heaviest ICE: 3663 lbs
Lightest hybrid: 3692 lbs
Lightest plugin hybrid: 4115 lbs
I remember recovering dead 18650 cells from laptop batteries and "restoring" them with a 12V modded PC PSU. Quite a few of them actually started working again and had some capacity for a few tens of additional cycles. Those cells were never left unattended in a charger and they were always only used in a device you could chuck in a moment's notice.
10/10 do not recommend.