I'm going to digress a bit because you write "Nobody has every published on that niche topic before".
First, let's dissect why editors might fail to find reviewers. When editors invite reviewers, the four most common results are:
- No response from reviewer
- Reviewer responds saying no time
- Reviewer responds saying the paper is out of their expertise
- Reviewer agrees to review.
Given that you're dealing with the top journal of your field, the probability of #1 happening decreases somewhat. After all, everyone wants to be able to say they have reviewed for the top journal before. However, it still will happen (e.g. if the invitation is marked as spam, if the reviewer is on long leave and not checking emails, etc.) If this is the root cause of the editor not finding reviewers, from the editor's perspective, the solution is straightforward - work harder.
#2 cannot be helped; some percentage of reviewers simply will not be available (tenure review coming up, funding deadline approaching, heavy teaching load ...). However, like #1, working harder solves this problem. As long as one is identifying the right reviewers, someone will eventually be available.
Finally, #3 will also happen some of the time. The probability decreases the more meticulous the editor is, but it will still happen, especially when dealing with a paper that is out of the editor's expertise. For example, suppose there's a paper that uses method X to investigate the effect on material Y due to electrons produced by natural process Z. Do you invite a reviewer with expertise in X, Y, or Z? Ideally the reviewer knows all three, of course, but if you can't find any reviewers with expertise in all three (you probably can't; if you could then the paper would be non-novel), then it's very hard to tell which is more important. Unlike the other two, working harder does not solve this problem. If Y is most important and you keep inviting reviewers who know X, you'll just end up with a pile of "decline to review" emails and get nowhere.
Since you have said "Nobody has every published on that niche topic before", I would guess that your editor is encountering #3.
Given enough time (and enough reviewers who decline), they should eventually figure out what kind of reviewer they should look for, but until then perhaps you can help. Instead of sending them a list of potential reviewers, send them a list of what expertise is required to review your paper. Describe what you did in non-technical language, maybe indicate which fields are likely to be most impacted by your results or are most likely to be most interested, etc. (Something similar to this should already be in your Introduction.) If the editor knows what kind of reviewer they're looking for, all that's left is to put in the time to invite them.