It is not necessary but you won't make much progress without one. Spiritual practice without a teacher is like navigating without a map, you may end up in America thinking it's India.
If you are somewhat serious about spiritual practice, of course you should find someone who walked some of the path already, otherwise it's impossible for you to know - by definition, you haven't walked the path yet so you don't really know.
At this point we are basically nitpicking over the Internet, but even if you want to take a journey just for the sake of the trip, a map is still very useful.
Most of the human endeavors (art, science, etc) are facilitated by having teachers. Spiritual advancement is no different and in my book, anyone who thinks different is delusional. You may be perfectly happy with no teacher, but you are leaving out a huge chunk of it by navigating on your own. Of course you are free to disagree, but I think it is essential to know about the range of possibilities before deciding if it's worth to have a teacher, and most people simply have no idea.
I’ve listened to a lot of Watts, which isn’t to say I understand it but rather I can recall it. your p1 concerns making progress, p2 on being serious. Watts talks on progress being an illusion, that the deer hunter doesn’t see the mountains. He also talks the same in being serious, rather than sincere, in that it’s like being an oak that cannot away in the wind, constantly concerned with holding yourself together lest you might accidentally fall through the ground.
By trying for America while worrying about India, would be like being hung up on trying not to be hung up. Or competing about who has attainted the most enlightenment, who sat for longest or the most uncomfortably. ~it cannot be grasped by thinking, it cannot be perceived by not thinking. ~The goose flying over the lake doesn’t mean to cast a reflection, and the lake has no mind to retain it. But still, it happens.
“I am not a guru, that means also that I’m not trying to help you or improve you. I accept you as you are. I am not out there to save the world.
[…]
And so I’m often asked the question, is it really necessary to have a guru? I can answer that only by saying, yes, it is necessary if you think so.”
https://alanwatts.org/searchable/1-3-4-spiritual-authority-p...
If you need to some authority with a how to book and various rules to force you to maintain some esoteric rules, go ahead. But it’s not necessary.