Your agent is wrong. If your visa was legitimately obtained then there won’t be any problems. If it was obtained using scam documents and bribes then yes, there might be issues.
Multiple Entry tourist visas are rarely granted even though it’s possible to apply for them. Since you prepay with the application the enhanced fee is usually lost. Plenty of reports here where people applied successfully in the UK.
I’d rather jump off a bridge than spend more than 2-3 months in one stretch in Thailand. It was horrible enough during the pandemic but hey, different strokes for different folks. I need a bit of civilization every now and then (HK, Korea, Japan etc).
Why would anyone even bother with that? Why not just go somewhere for a few days and then come back? I will never understand these people who want to spend the entire year in Thailand of all places and never travel anywhere. Are you on some Interpol watchlist? 😅
If you are the owner of the new company and have a registration already then you’re legally permitted to certify yourself anything you want. You don’t need a verified income, just a bank statement with 500,000 THB equivalent.