I’ve got Laptop 16, so wouldn’t comment on others, but
If you had the battery for 4 years, it’s definitely worthwhile to exchange it as it’s probably worn out. You should check the maximum charge capacity reported by the battery and if it’s lower than 80%, it’s time to change it since that’s roughly when the batteries start holding significantly less charge.
I wouldn’t even call it an upgrade, more a maintenance/replacement of wear parts
To whether the larger battery option is better - if you keep the laptop plugged in most (95%+) of the time, then the larger battery chemistry seems to handle this behavior worse than smaller capacity one. If you use the laptop off-charger at least once every two weeks, it’s fine to go with the larger battery and it should give you a bit more time. (This is my conjecture based on reading the forum and overall knowledge about batteries, other people might provide different guidelines.)
As far as the trackpad situation goes, there’s a few guides on the forum about leveling out your trackpad so it rests flat. I do find that my trackpad is bad about clicks in the corners, but the closer to the center you go, the more responsive it is. I don’t think there’s any trackpad upgrades per se, but a new input cover might help.
Processors…I use a 7840U and I’m very pleased with it, but even a 7640U would probably do the trick coming from an 11th gen. Could even try their Ryzen AI models. If you need Intel then the Core Ultra offerings are perfectly serviceable.
I agree with what @Gmanny was saying about the battery for sure. If yours still has the majority of its capacity and low cycle count just stick with it and know that it is an easy fix.
Any of the Intel Ultra or AMD AI mainboards will be a pretty significant upgrade coming from the 11th gen. Both of these will need the DDR5 memory. Personally I would just decide between 32GB or 64GB of memory.
The old mainboard could be repurposed in the Cooler Master case as a standalone little server or PC for someone.
If the laptop has the original case, there was an upgarde for the hinges and they made a CNC top cover for a little more rigidity. If neither one of those have been issues, nothing to worry about.
If it still has the glossy display the matte one is a nice change OR there is an upgrade to a fancier higher resolution higher refresh display that will slot right in.
Nothing in the trackpad department but a new one might behave differently. I believe it is easier to just upgrade the whole keyboard cover as an assembly if I remember from peoples stories in the past. It is late right now so my memory is fading.