How to (gently) pressure a collaborator?
You mentioned that you explained your situation to your collaborator many times, but you didn't mention the most important part: after you explain your situation, you ask for something specific and actionable, with a specific deadline, and you listen to what they have to say.
Outside pressure requires leverage; internal pressure requires agreement. The situation is asymmetric, as you describe - you need the publication, they don't. Nothing you can do really changes the difference in your needs. However, they hopefully have their own interests: they might be interested in helping you in your quest at this crucial time in your early career, and they might look forward to continuing your collaboration (hopefully they've enjoyed working with you).
The question is: what are you asking for, and are they willing to agree to the plan or not? There needs to be a clear answer here, an unambiguous commitment, and you also need to start considering a plan B if they are simply unwilling to agree or follow-through.
I would suggest an in-person meeting if at all possible, or at least video-chat. You can explain, but most importantly you have to make it clear that there is a real deadline you can't put off. For instance: "we would need to submit this by Date X, as I have to change my focus to application deadlines, and won't be able to work further on this. I think we've done great work, and it would greatly add to my own chances of getting a permanent position, so I really don't want to just let this just die, and I'd like to work with you on more things in the future after I get a position, but personally I either have to push towards publication by the deadline or just give up on it."
You should ask how they view the situation. Is this just something that's been put off to continue to improve because there was no reason to rush, or do they have some more fundamental concern about publication? Acknowledge their concerns, and try to find common ground. I doubt they want to lose you as a collaborator entirely, and if they can just keep polishing and putting things off they could very well do so indefinitely.
Most importantly, you need to come to a crystal-clear mutual agreement on a plan. If they just aren't willing to, so be it - at least you know and you can switch focus to Plan B. But if they can, then follow up in writing with the plan you both agreed to and thanking them for working with you on this, etc. They might still fall through - not all collaborations end up as productive ones - so you should still consider that Plan B anyway (especially as rejection is still the most likely initial result, but obviously "submitted" or "under review" is better than "in preparation").
You tried to persuade him already many times, to no avail. Now you need to bargain. Explain once more that you're under publication pressure, and that you believe your paper is long ready for submission​. But now also mention that you will have no other choice than to pull out of the collaboration soon and to publish your part of the work separately as a single-authored paper. This of course presupposes that your contributions are separable and that you are prepared to upset your co-author. (Regarding the latter: he upset you already and after this experience you probably don't count on working with him in the future anyway.)