Solo trip to Portugal does this sound like a good plan? Please give me your opinion and insight Portuguese locals :-)

This is my current plan (if you have any improvements and recommendations please let me know)

I land in Lisbon [spend a day or so]exploring

Then go up to Sitra (30 min train ride) then back to Lisbon OR straight to Lagos??

Then to Algarve (by train) spend a few days there then back up to Lisbon to catch my flight home

(total trip approx 5-7 days)

1) Do you think this is a sensible plan as in is it achievable especially during Covid-19 times?

2) How would you go about booking train tickets do you know any cheap ways to get around?

Thank you so much I really appreciate it