hello hannah - i am guessing that by saying ‘external memory / graphics card like the mac mini’ you are referring to a desktop device. in the apple system, this device could be a mac mini, mac studio or mac pro. while these devices tend to have better specs than some laptops, that’s not necessarily always true.
i am guessing that your current laptop is a 2017 apple laptop? if so, you would definitely get a boost by buying new apple device with the M series chip. You could consider getting a Mac Mini (I would suggest at least a M2 Pro configuration) or a Mac Studio, depending on your budget. Keep in mind that for these devices you would also need to buy a monitor.
Please keep in mind that while these devices can definitely run Twinmotion, you won’t have access to the Pathtracing option. For this, you would need a PC with dedicated graphics card. I would suggest you take a look at some desktop PC’s as well - most probably you can find a pre-built PC computer to buy at a shop near you. This could for example be your dedicated rendering machine for the office.
For another user wondering whether to buy PC or Mac, have a look at this thread: How much better is it to buy a PC instead of a MAC to use Twinmotion? (SKETCUHP 3D Renders) considering that I was always a Mac user. - #9 by Jeff_Harkey