How to work out what specs I need

@franfielding  You probably just need a gaming laptop, or more specifically, a dedicated graphics card, and these only show up in gaming laptops (and some lightweight, expensive non-gaming models).  The fastest integrated chips can run all Sims 4 packs together on medium-high or high settings, depending on playstyle and the player's tolerance for low framerates.  But if you want to add custom content that's more demanding than the game itself, this level of performance is probably not what you're looking for.

On that note, any current dedicated graphics card can run all Sims 4 packs together on ultra settings.  So even if you're adding a giant pile of cc, the laptop should have no trouble with high settings.  The cheapest gaming laptops, those with an Nvidia GTX 1650, generally start around $650 U.S. if you know how to look; models with the newer and faster RTX 3050 can be as cheap as $700.  Prices vary in other countries, but there are usually some reasonable deals to be found.

If you don't want a gaming laptop or would prefer not to spend that much money, you may want to wait for the next generation of integrated graphics chips.  AMD's next-gen options are supposed to be significantly faster than the current ones.  The 6000-series is technically released but not widely available, although that should happen soon considering the 7000-series is coming too.  Looking at benchmarks, the graphics chip in a Ryzen 5 6600U is about 30% faster in gaming than the fastest option from the last generation (the Intel G7 with 96 EUs, found in the i7-1165G7, among others).  The chip in a Ryzen 7 6800U is close to twice as fast and not all that far off a 1650.  All this is according to benchmarks; I haven't seen any results from Sims 4 players.