What part specifically within the plus plan do you need? You can have multiple locations all under the free square plan and then set each up for pickup via fulfilment settings, last I knew. We had 5 locations up until the fall of last year on the free tier plan, as we do 3 farmers markets a week, so it benefited us for reporting, but we ended up cutting it down to one to consolidate.
I don't disagree with the cost; it is absolutely a lot when that's all you are trying to do. One option we do is just have them put in the notes which market they would like it picked up at, and the date.
The only other thing I can suggest is that if the custom domain is keeping you from switching to free to accomplish this, then I would look into a domain redirect. This would allow you to use the custom domain, and then it would just forward to the free Square site URL. This would allow you to advertise your custom domain and allow you to drop to the free plan and add your locations.
I will add that not every registrar can do redirects, but I believe all the main ones do (Namecheap, GoDaddy, Porkbun, etc.). If you bought it through square themselves, then I don't think you could. So it all depends on who you got it from.
