Is the greater sin in lying and concealing a transgression, or in openly committing it and revealing Allah's concealment? And is it better to continue lying and concealing sins until sincere repentance, or to confess the sin to one's wife?

You have done well by not despairing of repentance. You should review some fatwas that help in abandoning masturbation. Whoever is afflicted by the unlawful should conceal it. If your wife asks you, use allusions. If lying is the only option, then there is no harm.