Hey folks!

I'm looking to move from my cyclocross bike to an e-bike for my commuter. I have a pretty awful knee, and while it doesn't always bother me while I ride, on a commute to the local bus/train the hills can occasionally get damn uncomfortable and turn into stopping to walk my bike up, or having to pedal with a single leg while I stretch and move the other on flats.

So I'm looking to an e-bike as the option. As someone heavy on the DIY and with a strong preference against proprietary, I'm hopeful to find something that:

• Has an easily replaceable battery, bonus for an assembly that I can rebuild with new 18650s or whatever, and a plug-in style connection for ease. Lets head off questions here by saying this would not be my first battery pack build, and I've got dozens of 18650's on hand.
• Is flexible as a commuter. I have commuted with my cyclocross bike all through the winter, which wasn't terrible with road tires but did have me switching wheels out. I wouldn't mind fat, I'm not out there for very long distance or speed anymore. Maybe one day I'll be up for a century again, but thats real low on my list.
• Has utility - one of the reasons I went cyclocross for my commuter was having a lot of mount points. I don't need a daily cargo haul here, but I like my mount points for flexibility.
• Has a long lifespan with maintenance and repairs. I've got bikes from the 70's and 80's that are still doing well (though I wish my wife liked the Fuji more, she is more comfortable in a hybrid style than road though). I want a frame that lasts, a motor that can be replaced without losing my marbles, I don't have any interest in built in lights that are just going to break and become useless because its all sealed, etc.

I think what I'd want is a combo pedal assist and throttle as an option, and a cadence sensor. A torque sensor I think would be more of an issue for my knee. If I can adjust the assist or just turn it off, I think thats the ideal, but welcome some input here.

Folding is a non-issue, I have a place for the bike to go and would want to take the battery with me to charge.

In terms of budget, I'll call it mid-range ($2k-ish) but if there is a fantastic option at $3500 I'm fine with that. Likewise, I'm not going to stick my nose up at an $800 option if all I need to do is add a few things or upgrade a motor if it ticks all the other boxes.

Any recommendations worth looking at would be welcome!

Edit: Northeast US for the record