An ATmega bootloader has no impact on power consumption once control is transferred to the main program / sketch. At that point the bootloader no longer has any role in operating the chip, and is just data passively sitting in (code memory) flash.
It’s possible to disable receive windows but perhaps a bad idea.
First, LoRaWAN is “transmit mostly” but is really designed to use occasional receives for important purposes. Even if you don’t use OTAA, you should still try to use ADR (unless your nodes and gateway never move and you manually tune the data rate and power).
Next, LMiC’s “what should I do next” logic is a bit complex, so ripping this out will require spending a fair amount of time to understand how it works.
Whatever you do, please don’t send uplink packets which imply they should result in a downlink, and then not bother to receive that downlink. It’s far more “expensive” for the network to send a downlink, than it is for you to receive it - because transmitting a downlink wipes out all 8 channels of the gateway, unlike uplinking which occupies one, and to an extent only partially.
If you really want a transmit-only node, you might do better finding a simplified codebase that just does that, and doesn’t fully implement LoRaWAN. I think there was one from Adafruit mentioned here recently.
Also, as an aside, some LMiC repos have a bug where if you try to use ADR without OTAA, and ADR fails, it will get confused and get stuck in an eternal loop mistakenly thinking it should do an OTAA join. I believe that’s fixed in MCCI LMiC, not sure about others.