You may be able to leave the entire joist alone, 100% intact.
1. The tub has both an overflow hole and a drain hole. Pipes under the tub can be routed so that the drain pipe goes down either right under the overflow OR right under the drain. That gives you a couple inches leeway right there before you read on. You may have to buy a fitting or two. Call the tub mannufacturer and look on their web site.
2. You can also send the pipe down through the floor anywhere else too. In the space between the overflow and the tub drain, or anywhere else.
3. Tailpiece can go sideways too.
You can send the tub tailpiece horizontally ("sideways") instead of vertically ("down"), right past the overflow standpipe, and then route it the way you want to after you get far enough away from the joist, turning down through the subfloor where it works best.
4. Zigzag offset; won't cut much of the joist.
You can buy offset drains, that go 45 degrees through your subfloor and that barely need to cut the joist. Many are 2" in diameter, made for shower drains. You can also build your own using "street 45" fittings if you want to, and then you could stay with your 1.5" diameter pipe. Copper fittings take less space than PVC; that may be useful, if you connect to the tub tailpiece with copper to get through the floor, and then once through you adapt to PVC (assuming your house has PVC drains). Search on "street ell" and "Street elbow", here and on the web in general. Plan, calculate, measure including the thickness of your subfloor and the height of open space under the tub too.
BTW, the distance you are talking about, may be the OTHER distance in a flat plane and not the distance halfway between a 32 inch wide tub. This distance is the most important one. It is always normal for the drain to be halfway between two sides, when the tub is symmetrical on that axis.
So, the distance that is always unknown and will vary from tub to tub, is the number of inches to the end wall, not side-to-side between the two sides that are 32 inches apart.