Thanks for the in general positive feedback.
Regarding battery behavior - let's start first with some generic statements:
When you say, you come from Stock ROM, this means Android 5.0.2 (aka API level 21) or even Android 4.4.4 (aka API level 20). LineageOS 17.1 is Android 10 (aka API level 29) - each Android generation in general is more resource-demanding than its predecessor, which obviously also impacts the battery behavior. Same as for the opensource-parts of Android, this also applies especially to the Google-apps, which also have grown over time and are for sure not really known for their energy efficiency (the Google Play services for Android 5 are significantly lighter than those for Android 10).
Custom ROMs consist of an Opensource part, but they also make use of the device manufacturer's closed source drivers (so called vendor blobs). So specific energy optimizations depend on the collaboration of specific kernel tweaks (open source) with the closed source device drivers. Whilst there are manufacturers being developer friendly, others aren't by means of really sharing their tweaks. And Sony is "not that developer friendly", even if one could get a different impression, when looking at their open devices program... - There are specific examples, where Sony even implemented things in their closed source parts, which didn't truly have a technical meaning other than making it more difficult for Custom ROM developers... (e.g. Camera)...
Talking about vendor blobs, the last vendor blobs from Sony for this device, which are still in use, come from 2015 and were designed for Android 5.0.2 and the Android 10 operating system has to work with them, which also isn't really ideal.
So from that generic stand point, I am not surprised, that the latest Stock ROM has shown a better battery behavior than this ROM. So in the case of older devices like this one, you indeed have often only the choice between an outdated Stock ROM with an outdated Android version, which does not provide basic privacy features and hasn't received any security update since years, but is lightweight, and a modern, but meanwhile heavier Custom ROM.
I'm not saying, that there isn't any potential for improvement, but neither @115ek nor myself are really "kernel/device cracks" being able to optimize everything - we take advantage of the work of very good developers, who managed to get the Z1c working with modern Android versions. Nevertheless, based on the above stated, I am pretty sure that it won't be possible to fully get to the level of the Stock ROM.