I don't think this upcoming fight will be his last one.
Because if there are offers that are too good on the table, he may want to fight again.
He admitted that he really does love boxing, so I think, sometimes he is just fighting for the love of it and not really the money that he can get from it.
He knows his body more than anyone else, so retiring options will really be up to him.
Granting that if he wants to fight after he wins in the next election, he'll probably run as vice president, imagine a vice president of one country fighting in the ring, I don't think his constituents will want him to fight he is the second powerful man in the Philippines, even if he wants to, his position will not let him, he should choose between fighting and serving his country, you cannot serve two masters.
On the contrary, if he run for Vice-President, I think he will have less task than compare to a Senator. Philippine politics is different though, and I would say that a VP is not that powerful so say the least.
And Manny has been doing this for years, boxing and then politics, so I don't see any problem if he is ever to be elected as public official again, he can manage his time. And with that, I highly doubt that he will retire if even he win in the next election. But probably he will have his last fight, either Crawford or Spence.
Manny will use this good opportunity to run for president, he is still active in boxing, so his popularity is still here. he already stated that he will run for president, according to one of his interviews, he started boxing as 4 to 10 rounders and aim to be a world champion, the same principle is applied in politics, he also want to reach the top which is the presidency.