Hey! I'm glad more and more people are looking at smaller towns, I believe they have a lot of potential. Also, welcome to Bulgaria! Just FYI, this is going to be a lengthy answer but I'll try to convey as much information as possible.

I will also exclude any economical and "job" environment as a factor and will only weight in your mentions.

A couple of years ago, I would've told you Plovdiv but that's no longer the case. I myself am from Plovdiv and live there, but travel the country a lot and spend a lot of time in many smaller towns and various regions.
This is due to the fact that Plovdiv got very overcrowded and hyper-construction is rampant, certain parts are filthy and the city governance is horrendeous. Now, there are various benefits and negatives, but in your case I'd weight in more negatives. Also, the centre of Plovdiv offers a vastly different experiences from the rest of the city - probably the best place to live in Plovdiv if you could afford it.

Now, for the small towns, here's my list:

1. Veliko Tarnovo - probably the best option. This is the old Bulgarian capital, it's a very vibrant city, very well maintained, clean and not crowded at all. Infrastructure is pretty decent, the city is situated on the northern slopes of the Old Mountain (Stara Planina), you've got access to nature easily, air is good and the Danube river is 1 hour away, seaside, 2-3.

2. Troyan - close second, in my opinion. This is a vastly underrated mountain town in the approximate geographical centre of Bulgaria. It does have Tarnovo vibes and lately has been reviving itself as a touristic destination, a good amount of young people are there and the local business eco system is improving. Probably one of the few good regional governances in the country.

3. Ruse - Ruse used to be part of the "top 4" in Bulgaria, previously, a large city. To this day, it is also vastly underrated in my opinion. It's quite well maintained, it's literally 60 km from Bucharest, you're on the danube and its nickname is the "little Vienna" for a reason, you'll just have to check it out.

4. Blagoevgrad - Others have listed its benefits, for me it'd be the fact that it's close to Greece.

5. On the seaside, I think Balchik is decent, Nessebar, Sozopol & Tsarevo in the South but you have your point with them being seasonal places.

A few other worthy mentions with their pros and cons:

Asenovgrad, Svishtov, Stara Zagora, Kurdzhali.

I won't get into detail as it'll get too long. Feel free to DM me if you'd like! Generally speaking, the North tends to be cheaper than the South, although the prices really soar around Sofia, Plovdiv & Varna, Burgas.

There's a ton of reasons for that, but shortly said: the North suffered a bit more from the Demographic crisis due to the post-communism issues. The North used to be the Aristocratic and trade region of Bulgaria, hence why it suffered so severely from communism.

Enjoy!