Better idea:

Make the salary of an MP the exact same as the average wage of the country.

This incentivises MPs to fight for better conditions for everyone.

To close the loophole that you talked about; all other sources of income received by an MP (even from existing investments) is taxed at 99% - to prevent another loophole, this also applies to spouses and other close family members. For the duration of time that the person is an MP then their family are only allowed to earn exactly their salary

Further, to prevent corruption, MPs must have all their bank accounts be public knowledge. A completely transparent cashflow. All MPs are also banned from making or receiving cash payments, it must be digital.

Attempts to circumvent these rules, or any proof of corruption, will be tried as treason, and punishable with life imprisonment and complete and total asset forfeiture to the state

If this sounds like it's "a lot" then it should. "MP" should not be a "cushy job" it should be hard fucking work but it should also be seen as an honour. You should WANT to volunteer to give up a few years of your life and give up your privacy for the chance to help better your country and your fellows. It should be absolute shit while you're doing it, but it should be the sort of thing that makes you a fucking hero, because with all these restrictions you would KNOW that people are doing it for the right reasons.