At this point, unless you find an incredible deal on a 2nd Generation Apple Watch SE, it's probably worthwhile to wait and see what they release in September, unless you have a solid use case for one right away. But if you find one at an excellent price, it could be totally worthwhile to buy now and trade in later if you like what you see in September.
This is pure speculation, but Apple has tended to reuse much of the "guts" from previous generation watches for the SE. I would expect them to continue that and use the existing case design, including band attachment mechanism on any theoretical SE they might release this year. If anything does get a redesigned band mechanism (which itself is just a rumor) there's a really good chance it's just the Series watch and possibly the Ultra, but not the SE.