China has already signed a defense pact of sorts with the Solomon Islands. One draft had some interesting provisions like:
The draft agreement says China, with Honiara’s consent, can send police and armed forces to “protect the safety of Chinese personnel and major projects” and also carry out ship visits and replenish supplies. Solomon Islands can also request Chinese police or soldiers to maintain order.
The final text of the security agreement between the Solomon Islands and China is secret, which is pretty unusual in this day and age.
So one could argue they have already joined a China-led alliance, yet this didn't lead to an invasion by the "West", so analogy with Ukraine joining NATO is not quite par.
Of course, the provision to deploy Chinese troops for internal security in the Islands (if it still exists in the final treaty) has some parallel with the Australia-led RAMSI deployment, but this time from much more authoritarian state.
The Solomon Islands, while technically a democracy, have been rocked by violence between ethnic groups, just last fall; emergency powers were enacted, etc. So there's quite the possibility that a slide into authoritarianism could be backed by foreign troops. There's also a Chinese minority on the Islands, which has been the target of rioters. So what the Solomons Islands present government possibly agreed to is more like Russia obtaining an agreement to send troop to police Ukraine.
Furthermore, in 2019 a Chinese state-owned company tried to secure a 75-year lease for Tulagi island (which was part of the WWII Guadalcanal campaign, due its deep harbor). The effort was apparently blocked by Solomon Islands' legal system. I'm not aware of NATO countries trying to secure pieces of Ukraine that way "ahead of schedule". So the goals of China seem rather transparent in that region.
Finally, if you think that NATO=USA, then you could argue that it has no business being in Europe (pretending WW2 never happened the way it did, i.e. including the Normandy landings), but if you admit that NATO is also Poland or Romania these days, then they have a border with Ukraine too, just like Russia has. Of course, Russia claims Ukraine is part of the "Russian world", but geographically speaking both NATO and Russia border Ukraine nowadays. Whereas in the case of China vs Australia and the Solomon islands the map speaks for itself, and explains why the islands were critical in WW2 as they were on the supply routes from the US to Australia.
Clearly, China wants to project naval power far away (thousands of kilometers) from its shores "just in case" (also in the Atlantic). This is "neither here nor there" relative to what the US wants with bases elsewhere, e.g. in the Gulf, but it's not really comparable with the proximity situation that Ukraine has relative to European NATO countries.