3 phase vs 2 phase nema 34

[JAZZCNC]

This is the spec of the 3-phase Nema 34 motor I currently have for the slider – Nema 34 motor FM86128SJT03-FSL120.pdf

Ok well, that's a whopping 14mh which is common for Cheap Chinese motors and will certainly need a good amount of volts if you want speed but it could go either way on motor heating with 230V with it only being 7nm.
However, If you have the Drive already then you can connect it up and will soon see if it gets hot, for a short period of time this won't damage the motor or drive.

Would you recommend getting a different PSU for the 4 x DM542T drivers? Something with a lower 36V like this for example - https://www.omc-stepperonline.com/po...ice&order=DESC

Given the application and the fact it's probably not going to be moving very quickly then you will most likely be ok. However I would certainly test the output with a volt meter before connecting to the drive just in case it's putting out more than 48V and if possible adjust the output to it's lowest setting.
If more than 48v then I wouldn't use it as your very close to the limit.

Having looked at the specs of the Nema 34 motor (holding torque 7Nm), it seems like the suggested Driver may be overkill. What would you suggest using instead?

Any of the higher-powered drives allowing 80 - 100Vac will easily handle this job, even with high inductance motor, you'll just have less rpm than a lower inductance motor would provide given the same voltage. But the fact you probably won't need higher rpm's then it won't be a problem.
Then just use a toroidal transformer which gives the output you require. You could actually use a toroidal transformer for the high powered drives provided it's within the drives voltage range. This would lower the output voltage and lessen the motor heating.