Since you are only interested in writing the forms correctly, you might want to learn the endings as a short mnemonic, just like one often learns days, months, numbers, etc.

You need to memorize "ons, ez, ent". Just learn the sequence however you prefer. When you need to actually write the verbs, you get the sequence from your memory, check which one you need here, and then write that.

With repetition this will become automatic. First you will have a good guess about which it will be, but still need some active work to check. After sufficient repetition you no longer need to check, as the initial guesses are almost always right.

Note that you do not use this process when speaking. You never pronounce the words themselves in English. You remember that you need an ending and then recover the correct one.

Personally, I remember that the last of the endings is "ent" by reference to Tolkien. The way you remember them can be highly idiosyncratic.

An alternative, not contradictory

An alternative is to get sufficient exposure to written French that the wrong options starts looking inherently wrong to you. Read a lot and write a lot. Maybe use flashcards or other tools as suggested by Tsundoku.

I would strongly recommend learning the correct pronunciation from the start instead of a pronunciation based on a different language, for several reasons:

• You will later need to invest time and effort to unlearn all the incorrect pronunciations. This time is better spent on something more productive.
• You might get confused when using listening materials that use the correct pronunciation.
• You might start using the incorrect pronunciation in conversations, which can lead to confusion. (People who aren't teachers won't correct you, which helps solidify the incorrect pronunciation.)

For my recommendations for learning verb conjugations, see the question How to learn conjugations?. In addition to what I said there about flash cards, you can also create flash cards containing short audio recordings of phrases such as "nous mangeons" (e.g. from Forvo), which you would then need to write out. Spaced repetition of this type of "mini dictations" would help you learn the correct spelling and pronunciation.