Not reasonably

Nothing you can build in vanilla Minecraft is indestructible.

That said, Simply Sarc recently posted a video on his channel, where he shows off a way to actually make a base indestructible, using 60 Elder Guardians, which are synched up so that one of them uses his curse every second. That means thats any player in the vicinity can't not have Mining Fatigue III for more than a second at a time.

This doesn't make your base truly indestructible, but depending on the material you used for building it, it will make breaking into a base take an awfully long amount of time.

Of course, your team would also be unable to break things inside your base, but you could have a redstone-operated door which can only be opened from the inside.

In pure survival, your team would need to capture these guardians manually, which involves raiding at least 20 ocean monuments. It is also possible to synchronize the curse timers, although the video doesn't go into details, mostly because ot would be utterly ridiculous to even attempt this method of base protection.

Don't use your main server for tests, even a small one

Using other worlds on your main server for tests is a really bad idea. Firstly, it does not give you a fully independent test/building environment (many values, like time, are changed globally, server resets are done globally, etc.). Secondly, if you break something on a test/building server, it doesn't damage the main one, while a test world might make the damage also touch your main environment.

Thirdly, a separate test server allows you to give full creative mode permissions to all the players, while it is rather dangerous on your main one, even if you think that you can really trust everyone who plays with you. Yes, your permission management plugins can malfunction, and I have seen Bad Things Happening because of that.

Once you have learned to set up one dedicated server, setting up another one is exactly Ctrl+C, Ctrl+V, and some changes in the server.properties file (Port, server name).

World Edit is a plugin that allows you to easily move structures from one place to another after you set it up.

Whenever I need to test something in Minecraft, I simply set up a testing server, then stop and/or delete it when it is no longer needed.

Of course, you can only use this solution if your hardware can handle two servers at the same time.

My answer is true even for a small server for a couple of friends, but is especially needed on any server that hosts more than, like, 7 people. Playing Creative and playing Survival are basically two different games, and it is better if they are played on two different servers that borrow each others' results when needed.

Having two distinct servers also solves a problem of moving stuff from one place to another. Players just log on to the server they choose at the moment, and all you need to move a structure on your main server is copy and paste a schematic via WorldEdit. Don't forget to check the amount of pricy blocks they use if you have an economy set up, for example, someone might try to sneak in a bunch of diamond blocks.