Getting it significantly lighter now is going to cost you a fair bit
it really doesn't always have to though. Especially if you've bought a base level bike
for instance
I changed out the wheelset and cassette and saved 1.2kg but overall cost came in at around £1200..
Can you list exactly what the component parts that cost you £1200 were?
and the OEM parts you took off?
You may actually be surprised at which component parts are significantly lighter.
eg. front 15x110 hubs are rarely any more than 20g difference between super expensive and entry level.
Even most budget spec rear 12x148 hubs these days come with sealed bearings and an alu freehub body so are no longer boat anchors but you might be able to save 90g if you're lucky by "upgrading" to a £300 hub.
OEM Eeb rims are often not much more than 550-600g each (and personally I'd be reluctant to go under 520g with the notable exception of DTs 571 or a Stans Flow). Yeah, you could go carbon, but they're rarely lighter and certainly not stronger (in some aspects)
Spokes. you're really unlikely to be wanting to use less than 32 spokes or anything lighter than DB 2.0/1.8 spokes so your only real choice is in reality is DB or plain guage. where there's a saving of around 25g per wheelset between the two.
nipples. alu nipple sare a third of the weight of brass but TBH you'd be an idiot to use Alu if you actually want to hold onto the wheels for any length of time. (Alu is soft and the threads prone to corrosion and siezing with the steel spokes and hence become a nightmare to true over time)
so what sort of weight is a £1200 wheelset really likely to save you over most OEM wheelsets? 200g? Yeah. probably/
You can lose 1.2g from a lot of OEM Eeb wheel package for considerably less than £1200
The biggest weight loss comes from Tyres, tubes. (and this will also make the biggest difference to the bike's handling with massively improved acceleration and nimbleness.
2x DH (or E-bike specific) casing tyres will weigh around 2600g. and the thick tubes a lot of manufacturers spec can weigh upwards of 300g a piece
Swapping out heavy DH casing tyres (when let's face it you're not a pro level DH racer, and may rarely even ride rough long steep DH tracks) to Exo casing tyres alone can save 1kg
Swapping out those heavy tubes to 120g of tubeless sealant per tyre and you've probably saved yet another 400g
so that 1.2kg can easily be saved for under £100
You mentioned cassettes. I still find it odd that Ebikers crave super low ratio wide range cassettes when in reality most of them rarely ride any steeper climbs than normal mtbs do. If you ride normal mtb terrain you should ideally be able to cope just fine with LESS range and a SMALLER, Lighter, more durable cassette. Which BTW will be far cheaper and lower the long term running costs of your Ebike massively.
and you need strong on an eeb
This is a huge misconception
Stronger parts are no more a requirement on an Ebike than the equivalent normal bike. ie. a bike of similar geometry, travel and intended use. ie. XC, trail, enduro or DH. rideen by the same rider.
A heavy clumsy rider will obviously benefit from stronger parts than a light precise rider same as normal bikes.