What's a quality brand to use? Water based or oil based?

I think this is the ticket. Can apply with a pump up sprayer

https://www.readyseal.com/

Wood Defender, from Mansfield TX https://wooddefender.com/
I get it at master halco

40 years in a redwood house, always bought the #1 Consumer Reports pick, which lasted up to 12 years (7 is the average in Aggieland) using brush, roller & sprayer, as the spaces needed .... here's their 2024 ratings

Years ago FLOOD made an acid stain one could apply to cedar with only a pump up tank sprayer, which soaked in & turned the wood a nice honey color that lasted "forever" ... wondering if it still can be had
https://www.flood.com/products/wood-stain

Radio show "The Money Pit" will always advise Solid, never Transparent ...

PPG Proluxe great product

Piggyback type question, hope it doesn't derail.

Is there any reasonable way to go back to a stain after someone used the opaque paint type stain? Previous owners used it. I don't love it and it doesn't seem to last very long.

Maximus Johnson said:

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I think this is the ticket. Can apply with a pump up sprayer

https://www.readyseal.com/

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I went with this one in the redwood color, and our fence looks amazing.

SW Superdeck, Semi-trans, oil based. Great product!

Absolute said:

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Piggyback type question, hope it doesn't derail.

Is there any reasonable way to go back to a stain after someone used the opaque paint type stain? Previous owners used it. I don't love it and it doesn't seem to last very long.

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If by reasonable you mean a pressure washer and orbital sander, then yes.