I wouldn't bother with the upgrade unless your T440P is falling apart like my X220 is. You can put in a 4TB SSD pretty cheaply. If you want to do heavy transcoding or something like that, use a remote server rather than a high temperature, power hungry laptop.

The T14 gen 1 is pretty cheap (I got mine 2 years ago for $200), and gen 2 and maybe gen 3 should be getting cheap. I wouldn't really call those successors to the T440p which is more of a workstation. But it should at least match the CPU performance while using a fraction of the power, and let you have stuff like thunderbolt for eGPU. I guess there's the P14s which is basically the same thing but quadro GPU. The P73 looks like it's finally depreciating though, but it's kinda for a reason. Below 11th gen Intel kinda sucks. 11th gen still sucks, but it sucks a lot less.

Also take a look at the P1 gen 3. Gen 4 still holds onto it's value decently well, but the gen 3 seems to be falling off the cliff. For about the same price as a P73 it looks like you can get a newer P1 with a 10th gen CPU.

@CorrodedCranium if you can afford a 600$ computer, maybe Framework will be a decent choice?