Mac "mini toslink" headphone jack

If I plug my iMac into my DAC via the iMac headphone jack with a mini-toslink to standard toslink cable, will that work as expected?
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Yes. Just be aware that there are some issues with settings in the MAC. The MAC has a utility called Audio Midi where you control the sample rate and bit depth output of the MAC. This utility does a poor job of sample rate conversion. When you start iTunes it gets the sample rate from the existing Audio Midi utility settings. However for unknown reasons it can often get out of sync and then the audio Midi utility is doing sample rate conversion which sounds bad.

1) Set Audio Midi to 24bit and 96KHz before starting iTunes. Check regularly that Audio Midi has not reset to default of a lower sample rate. In this case your lower resolution audio files are upsampled by iTunes which does a good job.
2) If you own a high end DAC then you may be inclined to allow the DAC to do all the sample rate conversion (in theory it should do the best possible job and arguably better than iTunes.) Since you likely have different files with different resolution you ideally require the Audio Midi to change to the sample rate of the file you are playing - so MAC does no sample rate conversion. I recommend Bitperfect app for this - you just tell the app to control the output sample rate to match the iTunes file you are playing and it will do this on the fly - ensuring all sampler rate conversion is done by your DAC
3) If you use a Tidal then you can set it up to control the sample rate of your MAC to match the file being streamed. (If you have Bitperfect installed then you just need to toggle it to "disabled" in order to release control of Audio Midi to the Tidal desktop app. I don’t know how to do this from a Tidal browser so I recommend using the desktop application which runs independent of any web browser you choose)

Finally ensure that audio gain is 1 and there is no EQ or volume adjustment done in iTunes - this can be configured in iTunes settings - this ensures no modification of the digital music file happens before it reaches the DAC.

If you so desire you can spend hundreds of $ on several applications that mostly do what Bitperfect does but may also add other features to color the sound in a pleasant way. My recommendations are only for those who want to hear as close to exactly what is on the audio file as possible.
BTW, what format are the files on your imac encoded as?
I'm using "Audirvana Plus 3" on my iMac because I want to listen to DSD when available. Unfortunately, when using the toslink line, DSD ISO files will not play through the DAC. An USB connection will play DSD ISO files. Apple Lossless files have no problems over toslink. This must be my imagination but the toslink line does not sound as good as the USB line with Apple Lossless files. Toslink gives more of the "sound in a paper cup" impression.