Will I face immigration issues returning to Thailand for a short stay after a previous visit?

Why not just get a tourist visa to be sure instead of guessing? 🤔

How are you going to use an agent to get into the country

Based on the dates, you spent roughly 140–145 days in Thailand from April to August last year, followed by more than four months outside Thailand, including time in your home country. Your upcoming stay in January is short (about 12 days) with a confirmed onward flight to Hanoi and accommodation booked. This still gives you good grounds to be assessed as a genuine tourist. That said, your earlier stay does place you toward the upper end of what is normally accepted under the visa exemption system, so entry remains discretionary. As long as you can show your onward ticket, accommodation, and funds if asked, issues are unlikely, and using an agent would not normally be necessary in this situation.

I would have the three proofs 20k baht or equivalent in other currencies and flight out and accommodation

No issue

No, you've been gone for almost 5 months, so you're obviously NOT trying to live in Thailand. Those are the people they're trying to keep out.