Do I have to register or affiliate with a party before I vote in the primary?

No. A registered voter is not required to pre-register or take any steps towards affiliating themselves with a party before voting in a party’s primary election. (§162.003)  Additionally, when a person registers to vote in Texas, they do not register with any kind of party affiliation.

This is all on that same source that you accused the other user of not reading...

Here's the whole section you're selectively quoting

How does party affiliation work in Texas? In Texas, there are several main ways for a voter to affiliate with a party: by being accepted to vote in a party’s primary election, by taking the required oath at a party precinct convention, or by taking a party oath of affiliation generally (§§162.003, 162.006, 162.007). A voter’s affiliation with a party automatically expires at the end of each calendar year, which is December 31. (§162.010). A voter who has affiliated themselves with a party is ineligible to participate in the party affairs of another party during the same calendar year. (§§162.012, 162.013)

Emphasis added to show that there is no persistent, year-over-year affiliation (I emphasize this word because there is no party registration in Texas)

And the only way you can vote in a primary is to register with them

I have never registered with a party. There is no field for party affiliation in the online voter registration form

https://vrapp.sos.state.tx.us/index.asp

And the registration certificate doesn't include a party affiliation

https://www.votetexas.gov/register-to-vote/voter-registration-certificate.html

And a Non-potato quality picture

https://disabilityrightstx.org/en/handout/understanding-your-texas-voter-registration-certificate/

you can do it on the spot if you want, but once you do it, that’s it

Every primary election I've participated in the official who checks you in just asked which primary you want to vote in. And since voter registration in Texas does not track party affiliation you're going to have a rough time convincing me this counts as registering with the party.

The spam you're getting is simply because it's public record whether you voted in any election, including which primary you voted in. But it's not the same as other states recording your party affiliation as part of your voter registration, which Texas flat out does not do.

If you lived somewhere where the GOP candidate is going to get 65 percent of the vote basically no matter what, and in the GOP primary you had an incumbent, somewhat average, right wing conservative that you've seen work across the aisle in the state legislator ruining against a hard core maga nut job, anti vaccine, anti public schools, openly racist etc. Do you think registering as a Republican to vote for the incumbent in that primary sounds like an okay idea? You can still vote for the Democrat and make your voice heard there at the general election even though it's essentially guaranteed he loses.

Or is the argument against this that if the crazy maga guy wins the primary there's a slightly better chance the Democrat can win? I'd call it unlikely where I'm at, but could see tighter districts working that way.