BO Pass train travel logistics

Assuming you had the Berner Oberland Pass in hand, you could board a direct train from Basel to Interlaken Ost, but just pay the fare from Basel to Bern. When the conductor comes by between Basel and Bern, you show your Basel-Bern ticket. If the ticket inspector comes by between Bern and Interlaken, you show your B.O. Pass.

You will change trains at Interlaken Ost because the tracks from there to Lauterbrunnen are narrow gauge and sections require rack and pinion assist to climb steeper sections. Also, you must take care to board carriages marked "Lauterbrunnen" because half way along, the train splits and the other half goes to Grindelwald.

And you would not have to "book" the train. You just buy the ticket and hop on. That is the way trains work in Switzerland.

So as it turns out - when I did the math, using the half-fare card saves me about $150 more than using the half-fare + BO Pass or just BO Pass (though I guess boat rides on thunersee and brienzersee aren’t covered with half-fare).

How annoying is it to have to buy tickets each time with the half-fare card, rather than just get on with the BO pass? Is it worth the $150 more for the BO pass + half-fare

(though I guess boat rides on thunersee and brienzersee aren’t covered with half-fare).

Yes, the boats are also covered by the Half Fare Card.
example: Boat from Interlaken Ost to Brienz is 32 CHF without a Half Fare Card, but 16 CH with one.

Sorry that was confusing Sam - I just meant that the boat rides aren’t free like they are with the BO pass. Would you go just half-fare?

Also, I know everyone likes the Swiss Pass but we’re doing so much lift travel where the Swiss Pass only covers 50% that I think the BO Pass makes more sense (i.e. Oescheninsee, schynigge Platte, First, Mannlichen, Brienzer rothorn, etc.)

The B-O map of validity seems to indicate that boats on the Thunersee and Brienzersee are covered.
file:///C:/Users/Owner/Downloads/Geltungsbereich-Regional-Pass-2022%20(1).pdf

It sounds like you should get the 1-month Half Fare Card plus the 10-day B-O Pass. You save 140 CHF on the B-O Pass, so spending 120 CHF is a no-brainer. Plus, that will also help on the Zurich Airport-Basel leg if you are still going to Colmar, plus the Basel-Bern Leg and the Luzern-Zurich Airport leg.

So i buy the half-fare card first and then how do I buy the BO Pass with a half-fare discount? Is there some kind of code provided to enter and lower the price on the BO Passs?

If you get a half-fare card and then want to buy the BO Pass online, you just pick the "2nd Class Reduced" option instead of full price.
https://www.regionalpass-berneroberland.ch/en/prices-tickets/

When you click to purchase it, you select "Reduced*" in the Reduction drop-down menu.

Is there a better website than others to buy passes/half-fare card?

I always favor buying directly from the source when possible. For Swiss travel products, you can buy online or upon arrival if that suits you better.
The Half-Fare Card is available directly from SBB, the national rail system. https://www.sbb.ch/en/leisure-holidays/inspiration/international-guests/swiss-halffare-card.html
The Berner Oberland pass is administered through the BLS, a regional train operator in Bern and Central Switzerland. They are fully integrated with SBB, but offer their own products as well. The link I sent before is their official website for the BO pass. It contains a lot of helpful information about the pass as well as purchase links.

So here’s another dumb question…if I am using the half-fare card and book a trip that requires a train and a bus (and a cable car maybe), is that all included in one ticket or do I have to buy a train ticket and then when I get to the end of the line, buy a bus ticket and then buy a lift/cable car ticket for my final destination?

Don't know if this is too late, but in Switzerland, you buy a ticket from your start point to your destination. So if that includes a train to a bus to a cable lift, you just have one ticket that includes every transport mode along the way. Example, going from Interlaken to Gimmelwald its just one ticket that includes the train to Lauterbrunnen, the bus to Stechelberg, and the cable car to Gimmelwald.