Can I extend my stay in Thailand for 5 months with a METV?

You can get up to almost 9 months from the METV, but then you need to have come to Thailand right when you received it. Whether it is now still sufficient for a total of 8 depends on when you received it i.e. whether this was within a month before coming.

To make it work, you need to exit and re-enter shortly before (or on) the last day to receive 60 days from there, and then extend those by 30 if necessary.

During these two periods, if you go abroad for a trip, you'll need a re-entry permit to keep your stay active. You can try exemption but after six or seven nearly consecutive months on the METV, getting that is not straightforward. Get the re-entry.

You can extend every entry by 30 days as long as you exit and enter before the visa expires you’ll get 60+30and you can exit and enter without staying out of Thailand more than hours

Without knowing what the end date is on your visa, it's impossible to say if you can get 5 more months.

The way a METV works is that EVERY time you enter Thailand before the end date written on the visa, you will receive a 60 day entry stamp. On each of these entries you can apply one time for a 30 day extension.

So depending on when the end date of your visa is, you might be able to get 5 more months. The way you maximize time with a METV is you leave Thailand and return just before the end date of the visa issued to your by the Thai embassy. That way you can get a final 60 day stamp before the visa expires, and you then apply for a 30 day extension at your immigration office. That adds about 3 months to the end of your visa.