If I've been married to a Thai national for 5 years. Our age difference is 5 years. How difficult is it for me to work and remain legally in Thailand? Is it still restrictive even for those married to Thais or is that only for those who aren't?

You receive no discount for being married to a Thai, except your employer can have a smaller investment in his company (but most don't even know that). You will still require the same work permits and visa entry but if you have family income of 40k or more per month you can extend your stay on a yearly basis without it being linked directly to your employment (and having to leave within 7 days if employment ends).

If you have employment elsewhere and no in demand skills (hotel management/chief/skilled teacher) it would probably be best to make your living there and plan retirement here. But for many that is not an option for one reason or another and if you have to make a living here expect a number of obstacles.

Is it true that even without 40k a month income (and not much money in the bank, and not 50 yrs old either), I can get a 1 year multiple entry O visa at a Thai consulate abroad, just using my wife's I.D. card and our marriage certificate?

(Meaning, I wouldn't get it thru the immigration dept, but thru an embassy or consulate abroad).

Would there be less (financial) requirements if i did it at a Thai embassy abroad instead of at immigration? Which countries if ever?

You don't get visas through immigration, ever. So, yeah, it'd be much better to get it through an embassy our consulate.

You can get the visa you just mentioned quite easily without having to show any money at an honorary consulate in your home country. However, I hear this is hard to get within SE Asia these days at embassies.

If I've been married to a Thai national for 5 years. Our age difference is 5 years. How difficult is it for me to work and remain legally in Thailand? Is it still restrictive even for those married to Thais or is that only for those who aren't?

Get a Non-Immigrant "O" visa based on legal marriage to a Thai national.

Enter Thailand.

Get a 90 day permit of stay.

Get a job.

Get a work permit.

Earn money, pay tax etc....

Extend your permit of stay based on having a work permit.

Simple as that

Not quite.

You do not extend your stay on the basis of having a work permit. You extend either having a family income of 40k or meeting the income requirements of your nationality for employment based extension of stay. The work permit does not mean you can extend your stay beyond the normal 90 day visa entry.

The name suggests the OP is a woman.

Doesn't this make it much easier?

So I've been married for just over two years and I'm still living separate from my wife.