I don't think the bundle size will shrink too much, especially if we include the morph-style merge, which is somewhat complicated

yes, I think you could do a stripped down htmx that doesn't support a lot of the stuff htmx does, particularly life cycle events and more elaborate swapping mechanism

for example, most response headers could be just done as the HX-Trigger response header + some client side scripting

history support and form-value gathering is likewise a source of complexity and code that could be omitted or done less completely

Sounds like reasonable places to strip out stuff while keeping the fundamental features. I think the history support would be important enough to keep though.