I will be going to Mexico in April and am a bit confused as to what I will need when returning in May.  I have a retirement visa and a reentry permit.  I have the green vaccination certificate and my insurance covering Covid.  I think I will need PCR test before returning and stay in a hotel when arriving and 5 days later both for more PCR tests.  is this correct and is there anything else I need to do.

Apply for a test&go. 
you can do it just 10-14 days before return, when you know your exact day. 
by May requirements will likely change. 
probably no 5th day pcr. 
maybe even no 1st day pcr. 
 

The hotel stays on day one and day five, along with the two PCR tests, need to be prepaid in order to get the Test and Go.

They can be different hotels, but they must be government approved ones.

Since you're not returning until May, I would wait and see what the rules are 3 or 4 weeks before your return. I've completed two sandboxes and one test and go. In each case the rules that were in effect before I departed had been changed by the time I returned. 

23 hours ago, Tropicalevo said:

The hotel stays on day one and day five, along with the two PCR tests, need to be prepaid in order to get the Test and Go.

They can be different hotels, but they must be government approved ones.

Of course, brown envelopes on the go.

my company jsut confirmed that Test and Go applied if I travel out an in Thailand even if I've got working permit and related visa

IMHO, If there are no more variants to set the world back again, come end of March, restrictions will be a lot less for incoming to Thailand