Power up Confusion Exeggutor?

No, just keep it as it is. Keep saving for Tangrowth instead.

I would not power it up. I have a Confusion Exeggutor maxed and rarely use it because I have plenty of better grass and psychic attackers. It's still worth saving because Confusion is legacy and extremely powerful.

Confusion is good but guides here say Extrasensory is a close second. Eggs are not uncommon (currently even a T2 raid boss), surely a 90%+ Es would be as good, or better all round.

It was a great candidate back when there were fewer options. Now it's not necessary.

I would say it's no longer worth it, there a lot of better grass options now(sceptile/frenzy plant venusaur/solar beam groudon) and as far as psychic types go eggs is way down the tier list. It's nice to have but not as strong as he used to be and no longer worth the dust(that said I do have a maxed 98 Egg from way back when)

If you like to do some unique challenge, especially in this fighting event, it is a good option
If you are planing to use it as your main attacker, there are better pokemons in psychic department
For full grass it is still good, great option if you wanna use it together with other strong grass attacker like Venusaur with Frenzy Plant

Paired with Seed Bomb, it makes for a reasonable defender, but I would still personally save the dust.

I'm going to disagree with the group and say, power it up. I have 2 con/sob eggys that I use often for Machamp solos. I like that they are not so fragile like other psychics. You do lose some dodgability though.