You mention time building, but for what purpose? If you're not going beyond the CFI what are you building time toward? Your goals should drive your plan for building experience.
As for the rating, it certainly doesn't hurt to have ME on your certificate, but it's not necessarily helping anything either. I wouldn't break the piggy bank open to do it as there's a pretty low likelihood of using it unless you make a concerted effort to get on at a busy flight school, pay your dues, and do some multi-engine instruction. However, there's also a chance that with networking around the airport you might get to know someone and get invited to right seat a King Air or other smaller twin and with the rating you can log the time while flying... so ya never know.
Whether you get a MEL or a SES or even a tailwheel endorsement, with a few hours of training from checkride/endorsement preparation, then neither is going to be particularly useful as insurance isn't going to let you do much more than continue to ride along with an instructor anyway... but I'd say there's a hair more chance of networking into an opportunity in a multi around the average GA airport. With the SES you'd have to be pretty intentional to continue renting a SES to build some time and even then there's going to be limited opportunities to use it unless you buy one or instruct for a seaplane operation.
The way I look at it, as someone who flies mostly for the enjoyment of it (but still tries to get paid), is that if you need a flight review and have a little extra cash available, go add a rating, mostly just for fun and personal goals. So, go after personal goals while also being realistic about what opportunities might be available in the area.