You need to be clear with him - do you expect him to just have a plan for engagement agreed by 2 years, or are you expecting to be engaged by 2 years with plans for marriage?
If it's the former then he's only agreed to think about it and still feels he's got 6 more months to decide whether he wants to get married or not. In which case, you are pressuring him as 6 months is what he needs to make up his mind.
If it's the latter then you need to wait till he's proposed or you have proposed, or you've both just agreed to be engaged BEFORE you look at rings. Ring shopping is done after the agreement to get engaged. But if he doesn't think he's agreed to be engaged, just agreed to figure out if he wants to be engaged - you're jumping the gun on rings.
Ask him if he's ready to be engaged. If the answer is No, then you ask him how long he needs to make up his mind and what information does he need he doesn't already have. Then you decide if you can wait. If the answer is yes, ask him what specifically about ring shopping is pressure if he consider himself engaged.
I do think 2 years is enough time at this stage of life to know if you want to be married or not. But you're at 18 months, not 2 years and if he needs another 6 months to figure it out, and you think he's the one - what's the harm in waiting? 6 months is nothing in the span of a lifetime and marriage is a big decision, some people just need more time. But you cannot go ring shopping with him if he's not ready to be engaged. You can do it yourself or with friends, but he isn't in the headspace to engage with it. And you can't force it.