S-Pass Rejected

Sorry to hear. If you read in our Singapore forum, many  threads, we have advised numerous times that don't resign your job before your new employer applies work pass and it's approved. Cause getting a work pass in Singapore is now tougher than earlier.

Anyway, you have already did a big mistake which you will repent for now.

Back to your second query, we too have suggested that appeal without any material changes to original documents submission such as any salary and position changes (revised salary or position or both should come with valid reason or else MoM will see in suspicion) to earlier or submit additional certificate or any achievement, the result won't change. So, see if you or your new employer could do anything above to provide a valid appeal.

I can understand your situation but you should blame for yourself only. From Expat team, we wish & pray that this issue shouldn't draw negative attention towards your marriage. Tell them this is temporary, tomorrow you may be landed with a better job and opportunity. Good luck to you and your fiancée. Stay blessed!

In addition to what Surya wrote above, I think the rejection might be because of your change of job scope from manufacturing to mechanical design. If you never worked in design before, they might not consider you fit for the job. In that case, it could help to submit mopre details about your design knowledge or that you, maybe, had some design responsibilities in the past despite differing job titles.
And if all else fails: In case your wife-to-be is Singaporean (and has sufficient own income), you can always apply for an LTVP to live with her after the marriage - and with some luck you'd then also be allowed to work on LoC.