About a problem with finding potential referee for my manuscript
This kind of decision is always a balancing act with no obvious answer.

If you do not withdraw: Do note that the editor might not invite all the reviewers you suggested. That's the lazy thing to do, but they might also be doing their own checks, e.g. did you coauthor a paper in the last five years? Did you work at the same institution in the past five years? Et cetera. Remember there're systemic differences between author-suggested and editor-suggested reviewers; you cannot expect the editor to rely entirely on the reviewers you suggest.

If you do not withdraw, there's no way to tell how long it'll be before this journal finds reviewers. 

If you withdraw: in this case you'll probably wind up submitting to a different journal, which means you start from square zero, which means they are also going to be looking for reviewers. It's possible the new journal handles things quicker. Perhaps their editor is more motivated and/or has greater connections, perhaps they invite more reviewers, perhaps they send more reminders ... but it's also possible the new journal handles things worse. They might not even respond to your queries.

Ultimately nobody can tell you the answer, because nobody can predict the future. You'll have to make the decision yourself. Are you willing to give up what you've already reached in this journal for the uncertain prospects at another journal? Expressed differently, is a bird in hand worth two in the bush for you? It's a question only you can answer.