Technically you can get as many as you want. Some claim there is a limit, that really depends on the embassy that you use. I would definitely shop embassy (get a visa from one, then another etc). The METV is your best bet. They are valid for 6 months, but if you stamp-in on the last day of it's validity, it can be extended 3 more months giving you a total of 9 months.
When I was shopping for METV from Australia I found Brisbane and Melbourne were the friendliest embassies.. I think post covid Brisbane Thai Embassy is gone, not sure about Brisbane. Point is, different embassies have different policies.
If you are showing that you leave Thailand every so often, the Thai authorities won't care too much about you coming. They are more concerned about people who don't ever leave.
You're first long trip here will be the most confusing.. After you experience how things work, the visa process will make more sense... Thai authorities are goofy.
I have done back-to-back tourist visa for several years. In the past nine years I would estimate 60% of those are tourist visas. Many people in group say it's not possible but it seems to work for me.
If you are 50+ you can get a retirement visa. Otherwise habitual tourist visas. You can get multiple entry for either, this will save you a little hassle and a few dollars.
I pay 3500 for a visa run from Pattaya. I don't recall the name of the visa service, maybe First Class Visa (not sure). They operate out of the Victorian on Soi 6 Pattaya Beach Road. I don't know if you're in/near Pattaya.
Someone in this group will have a recommendation for whatever area you're in. Time/Distance will effect the cost.
If you want to sleep in the temple and eat only rice you can probably get by on a dollar a day. If you want to live in a high rise building an drive luxury cars it will cost considerable more.
Recently someone asked a similar question.. If 2000thb/day is a enough to get by. That seemed like a pretty good number.
I transfer 65,000 every month, of that, 20,000 for rent, 5000 for bills, leaves 1300thb/day. That's enough to get by but not enough for the incidentals, traveling for visa runs, travelling for recreation, lost cell phone, etc.
Bahtsold.com is a pretty good site to find rentals, to give you an idea about what you can rent.
I believe the best option is to get your retirement visa here. Come on a tourist visa then work out the retirement visa. You will want to ask the group this question because I am not so familiar with UK Citizens, but for US there are different insurance requirements and its more favorable to get the visa in Thailand.
You can survive comfortably on 40,000 but it won't leave you much to do other stuff.
Why bother with the ED visa? You can learn Thai, you can go to class, without the visa. The ED visa is fine but the last I knew, they require you to go to class and take tests to keep your visa. Some people have reported they've had issues because they had an ED visa. An immigration officer once told me ED visas after 40 look suspicious. That doesn't mean much but if you can do without a ED visa, why do the ED visa.
Multiple Entry Tourist Visa (METV) is a good option also. They allow you stay in Thailand up to 90 days at a time. Every 90 days you will need to exit Thailand but you can come back 10 minutes later.
Retirement Visa require you are 50+. If that is a couple of years away for you then you can get by on a couple of METVs. If you are several years away, you will have to get creative (including ED visa) but I find it's worth it to live here.