I’ve found WPA2/WPA3 mixed mode to be very hit and miss for a few legacy devices unfortunately. They can see the SSID, but they simply won’t connect even though it’s supposed to be ‘backwards compatible’.
I’ve basically done what was mentioned here and went with 2 SSIDs on my network. One for modern devices that uses WPA3 exclusively and allows for all 3 bands along with Wi-Fi 7 features such as MLO. And a second one for legacy devices which uses WPA2 exclusively and only supports the 2.4 GHz and 5 GHz bands.
Even then, Wi-Fi 6E devices are in an awkward middle ground. While they all support WPA3 and can use the modern SSID, they don’t support MLO so they can sometimes hang on to the 6 GHz band which drops off more quickly at a distance instead of switching to one of the other two bands when appropriate.
A laptop from 2015 probably has an Intel 4th or 5th gen CPU. If 8 GB of RAM is an issue, then you could upgrade it to 16 GB in theory, provided that it isn’t soldered down. That could keep it going for a bit longer if 8 GB is the only problem that keeps it from daily use. I’d say that 8 GB is about borderline these days. It’s still perfectly fine for basic web browsing but not much more.
Even though RAM prices are in the stratosphere, DDR3 prices aren’t as badly affected, so you could get 16 GB relatively cheaply. I’d look for 2x 8 GB DDR3L-1600 SO-DIMMs. Just ensure that the laptop can accept it.
Upgrading the hard drive to an SSD will make a huge difference for performance as well if that hasn’t been done already. Unfortunately, 2.5" SATA SSDs today are even more of a ripoff than M.2 SSDs at their current elevated prices because they aren’t really being produced anymore. At that point then yeah I agree with the sentiment that putting the money towards a new laptop will go much further.