Test series for mains - when to start and which ones actually help for UPSC answer writing practice

starting mains preparation properly this time. previous attempt i gave random tests without structure. need clarity on test series approach.

my situation:

upsc 2027 attempt. prelims in may 2027. mains in september 2027. currently have about 18 months.

confusion areas:

when to start test series:

some say start test series from day 1. learn by writing. others say complete syllabus first then practice. conflicting advice.

my worry: if i start tests early i wont have enough content. if i start late i wont have enough practice.

how many tests to give:

some toppers gave 100 plus tests. others gave only 20-30 sectional tests. no clarity on optimal number.

which test series:

famous ones are vision insights forum ias gs score. but fees are 15-20k each. cant afford multiple. need to choose wisely.

also heard coaching tests are often harder than actual upsc. is that good or bad for practice?

sectional vs full length:

should i do gs1 gs2 gs3 gs4 separately first? or jump to full length tests directly?

evaluation quality:

heard evaluation quality varies a lot. some tests have copy paste feedback. how to identify good evaluation?

specific questions:

1. for 2027 mains when should i give my first test? like which month?

2. how many sectional tests per paper recommended before starting full length?

3. between vision and insights which has better gs paper evaluation?

4. is it worth giving optional test series or just gs is enough?

5. should i attempt tests for papers i havent completed syllabus for? will that help learning?

6. how to analyze tests properly? just reading feedback or something more structured?

7. are there any good free or cheap test series for those with budget constraints?

8. how much gap between tests is ideal? weekly? fortnightly?

those who cleared mains please share your exact test series experience. especially what you would do differently if preparing again.