A word or a phrase for "completely optimised, thoroughly researched technology" [closed]

I am looking for a word or phrase which can be used in the sentence:

It is a rather old, but _________________ technology.

The word or phrase should address a technology that is totally investigated, researched in detail, and thoroughly optimised, so there is nothing more to find out or improve.

Solution 1:

Most common term I have seen repeatedly and would thus use is, well established.

It is rather old, but well-established technology

Note as correctly point out by @Chappo below in this case grammar dictates a hyphen is required since it is a compound adjective preceding a noun.

"science can be leading edge or well established" Wiki Technology

Solution 2:

I don't know of a technology that cannot be improved, but we often use the term mature to describe technology that's deemed developed enough to be left alone:

A mature technology is a technology that has been in use for long enough that most of its initial faults and inherent problems have been removed or reduced by further development. In some contexts, it may also refer to technology that has not seen widespread use, but whose scientific background is well understood.
Wikipedia

mature
6. No longer subject to great expansion or development. Used of an industry, market, or product.
American Heritage® Dictionary

Solution 3:

You may be looking for proven.

It is rather old, but proven technology.

Collins:

proven in British

adjective
3. tried; tested

a proven method

Collins English Dictionary. Copyright © Harper Collins Publishers

proven in American

adjective
2. known to be valid, effective, or genuine

a proven method

Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by Houghton Mifflin Harcourt. All rights reserved.

Solution 4:

tried and tested

adjective - recognized as reliable; found to be successful

tried-and-tested in British (ˈtraɪdəndˈtɛstɪd), tried-and-trusted (ˈtraɪdəndˈtrʌstɪd) or US and Canadian tried-and-true (ˈtraɪdəndˈtruː) adjective recognized as reliable; found to be successful

https://www.collinsdictionary.com/dictionary/english/tried-and-tested

As noted the expression varies somewhat according to location. All of the above versions would be understandable to a British person.