I think your question is about how (or if) you need proof of departure within 30days. I have used the 30day visa exempt entry method numerous times and have never been asked about a departure proof. In fact, my recent examples involve that I live here and had no proof of on-going plans; instead, I just got in on the 30day and reactivated my "live in Thailand" permit during that time without any departure papers.
Actually, all I have heard of are stories of "over stay" fines applied upon departures beyond the 30-day limit.... meaning that the only check on you will not come upon arrival but rather will be checked as you depart. Maybe incoming checks on onward plans are more often done nowdays at entry, but a forward room reservation should do that or other-nation visa date.
Nevertheless, if you fear and need onward travel proof, just get a printed room reservation in any country except Thailand that is after your 30-day stay. Otherwise, your advance visa for Laos, for example, is proof of your departure date plans. Yes, you can get a visa into Lao when you get to the border, but why not get one earlier?
Note that very recently, that madman Trump is making some noises about travel into Cambodia limits. How you can check what that loony Trump is doing as he changes hour by hour is a puzzle, but do be aware of reports about Cambodia travel.
Also note that you can not travel from Thailand to Vietnam by land without entering and crossing Lao PDR or Cambodia. I can not recommend such a land trip, but for the hardy and intrepid, I guess it can be ok. Any way, enjoy your visit !!