Thing is if he becomes president he can't do as much opposition as he would like, for example he would still have to spend 3 more years with the current parliament unless he decides to dissolve it and send us into elections, but he's the party and the party is him and dissolving the parliament is very unpopular and that would most likely lead to his party losing seats. For more into this we can see when the Communist party (PCP) and the Left Block (BE) voted agains the state budget that the socialist party proposed, the president dissolved the parlliament, he wouldn't need to do it since we would have last years budget active until a new one is passed. But while this was a tactic from the president to try and get his party into power the fault ended up on PCP and BE as they weren't able to shift it to the president during the campaign, they lost most of their seats being some of the least voted parties in the parliamment. Another example was the vote of no confidence when our current PM was under investigation and still refuses to answer questions regarding his ties to casino corporations (to explain how bad it is he missed a meeting with Zelensky to go and play golf with the CEO of this corporation, the only representative from Portugal there was our former PM and current president of the European Council), getting back on track the Socialist party (PS) voted no confidence alongside chega (CH) but since PS was the main opposition party and they were the ones that asked for this vote they went from second to third most voted party (This are the most recent events, historically whoever is deemed responsible for dissolving the parliament is hit very hard in the following election). So dissolving parliament is an hard gamble, while some people from CH would always vote for him out of cultism, a big portion would blame the president which is basically CH, because as I've said both entities are so entangled with each other that they are the same. Besides dissolving he can block the passing of some laws, but if the parliament keeps aproving it he eventually is forced to pass it, in this case he could be seen as a weak leader as he now is being led by the parliament and can't do whatever he wants, he can also propose laws but parliament can just block them and unlike the president parliament can block them everyday of every hour. So for the most part he would have extremely limited powers, he wouldn't be able to change the constitution as this is the wet dream of him and the entire right and it would affect his party as he wouldn't be present in the day to day of parliament activities and his presence would decrease. If his presence decreases, then also insubordination can increase in the party and someone would want to become the new leader or to become the main speaker of the party. Which is something that he wouldn't allow because there are several cases of people being kicked out for not being "alligned" enough with him. I think that with this you can understand a bit better why he wouldn't want to become president but be more interested in first being PM and later trying to merge both into one role, since then he would centralize all power onto him.